The magic in rooting in these small pots is that they are never transplanted to another pot and they just go into the ground when they are large enough plants. . With most all the other rooting procedures there is at least one move to another pot. They do get a bit root bound and grow bit tall but it has worked rather well for us over the last 20 years or so. If a cutting is being saved as pot tuber it is transplanted to a 4, or 5 or 6 or 7 inch pot. Mostly 4 inch pots for pot tubers. Halls of Heddon in the UK grow their pot tubers in 3 inch pots and I tried them without success. I have a couple hundred 3 inch pots I will never use that I need to give away. Halls of Heddon grow their pot tubers in the green house with automated watering and fertilizer and probably shade cloth when required. My outdoor growing area does not work with 3 inch pots although HH Black Beauty made some nice little pot tubers. 
Taking Dahlia Cuttings
Teddahlia Oh wow, how beautifully simple to skip the up potting step! What is it about the small-sized pots that cause them to grow taller? Or is this happening when/because they’re all crammed so close together in a tray together? Do the tubers eventually break out of the pot during the grow season, underground?
Is it common for cuttings that have been rooted and moved to a 4" pot to have the bottom leaves dry up? If not, what should be done to prevent this?
"...bottom leaves dry up." Bottom leaves are the oldest on the plant and often die. Many show gardeners always remove them. I do not, too much work and it causes the plants to grow taller and our plants are tall enough. At a trial garden they removed the bottom leaves from one of our entries and then again. and maybe third time.(fungus?) The plants were 11 feet tall and could not be judged as the flowers were too high on the plants.
I have a question about fertilizing cuttings. @Teddahlia has mentioned using a diluted mix of Jack's 20/20/20, which I found at my local place. Is this for the cuttings after they have roots? I seem to recall you suggested also giving it to the tubers that are providing the cuttings. Is that correct? I wonder if I was too stingy with water on those tubers.
Mine seem kind of slow--it's taking me a lot longer than I'd read to get sprouts and roots and growth. Though now they do seem to be picking up speed. Oddly, it's the most stubborn plant from last season that is giving me the best material. Orange Globe was fussy in storage, except for one beautiful tuber that I've been babying in hopes that I can try this one again. The blooms were gorgeous, but she was slow and short. It was practically impossible to purchase this year, so I really want to succeed with the cuttings.
Thanks in advance!
Half strength greenhouse fertilizer(Miracle Gro is 90% as good) is good for watering pot roots and the rooted cuttings after they root. Watering is an art and too little is better than too much but just right is best. If you can decipher that, you will know how to water.
Teddahlia Thank you for so deftly anticipating my questions - would “half strength” be 10-10-10? I guess I’m not familiar enough with greenhouse fertilizers to know whether they even make this at 10-10-10 strength. Your advice to go half strength is starting to make more sense 
And I will work to hone my pot-root watering skills asap! I feel like learning how to successfully water everything is a constant learning process for me, unfortunately. Hearing that erring on less vs more (if we’re choosing “one of two evils”) is very helpful!
The process I've been using -- spray bottle, nozzle pointed directly at edges of the pot and fairly close. Three water bursts to each side, so that the water drips down (well, for the most part...) the inside edge of the pot. Theory being that we want the roots to stretch out. I don't remove each pot from the tray, I just change the position of the tray so that I can do the next side. Good in theory and practice? What say you (all you more experienced growers...) 
Another question I have -- is it a wise procedure to finish off the newly potted up tuber with Vermiculite which would help keep the soil moist longer? I've just started doing this, but I want to make sure this is a wise move. My pots do seem to dry out rather quickly (I'm using 2 different types of LED - white and the dastardly 'full spectrum' which I hate). If this is a wise move, how deep would the Vermiculite need to be to be most effective? Just a splattering to cover the surface or an inch or more?! Thanks, all....
The mixing ratio for Peters Professional 20-20-20 General Purpose Fertilizer is 1 tsp per gallon of water. Half strength is 1/2 teaspoon per gallon. Jack's and Plant Marvel all come in 20-20-20 versions . I am trying Plantex Solutions 14-4-14 Cal P Mag Water Soluble Fertilizer that is used by our local 10 acre flower greenhouse company. They water everything with it.
What say y' all - would it be good to start a list of cultivars that don't seem to root well by cuttings? That way we could determine if we have poor stock because someone else meets with great success for that cultivar, or that selection just doesn't make good cuttings, because 99% of us meet with failures.
Teddahlia At a trial garden they removed the bottom leaves from one of our entries and then again. and maybe third time.(fungus?) The plants were 11 feet tall and could not be judged as the flowers were too high on the plants.
You were asking last season why there seemed to be so many comments about HH Dungeness growing into a ten foot tall plant (presumably yours aren’t as tall). Perhaps this is why - I definitely remove leaves up to about 18” off the ground once the plant is large enough. (And my Dungeness was one of the 10 footers).
Speaking of HH Dungeness: We have another birdnest dahlia that needs to be released and I will have lots of tubers of it next year and have lots this year. Margaret thinks it is not refined enough. I love it and it is only
8 feet tall when Dungeness is 9 feet tall.
I like that one too. I grew Hollyhill Dungeness and it is usually yellow for me till the fall when we cool down then goes more pink. Looking at my map I grew it beside sir paxton j which I also loved I think i had both marked as inverted cactus
I made a visual aid for the cutting terminology discussion. I struggled to put a label to the “Top cutting”. Is it just a nodal cutting? Or an apical (as opposed to lateral) cutting?
I also side-stepped the use of “node” on the leaf cuttings and opted for the term from @Bessie ‘s textbook: “Leaf-bud cutting”.
In Botany, they have more words to describe every part of a plant than Carter has pills(Carter's Little Liver Pills used to be a cure just about everything). Your terminology is very descriptive and the picture seals the deal. White cuttings can be slightly different as they can be the same as tuber cuttings but with no green color or they can be two nodes(one for roots and one for leaves) from a very long white sprout that has several immature nodes on it. I have mentioned that Dutch growers like white cuttings.