I see posts on as what Margaret would call “arranging” your dahlias in the garden to look good and this would be an interesting goal to make your dahlia flowers blend in artistic harmony. This thread is on hybridizing new dahlias and not primarily on making your garden look good. Having said that here are some things that we do that can contribute to a nice looking garden.
We plant many, many seedlings. One of the major parts of breeding is the evaluation process. They need to be grown well enough to give them a chance to impress you. Here are some things we try to do:
First year seedlings: Since we label all our seeds with their parent(s) name we try to arrange our first year garden into similar zones of form. Of course, Margaret's waterlilies are given their own space where possible. Last year we had an entire 50 foot row of waterlily seedlings and they got the number one prime position in the first year garden. It was a dream come true to be able to appraise them without crawling through crowded rows of seedlings and to be able stake up the best ones. In most years, we try to plant by form in the seedling garden and in one year before Margaret's waterlily mania we planted an entire row of miniature ball and similar types in that prime row. From a distance, the shorter plants covered with ball shaped flowers in a myriad of colors was spectacular. But as with most seedlings, they were not so nice close up except for few nice ones. And so it goes each year in the first year garden, some rows are the same type but when you grow so many and the plants develop at different times, it is usually a crowded mixture of all types reminding me a jungle of dahlias and if they were not so precious, one would use machete to chop your way through the rows.
Second year seedlings: This is where Margaret has declared that when I am planting the numerous rows of tubers and plants, I am not to plant two of the same color together. I am 90% accurate but every year I get an earful as she declares those colors do not belong together. Last year my mistake was three second year seedlings that were purple and even worse were flowers of a similar form. And they were all huge plants that grew both tall and wide. Oops, my penance in the end of the season was to be crawling on the ground checking plant tags and sorting out which ones were which and all were being retained. 
The second year garden is not as orderly as it could be. Generally, the waterlilies are together but most everything else is several plants of this one next to several plants of another one. This works somewhat well as most of the time there are 3-5 plants of each hopeful and they can look very good when there are several plants of a nice one but of course more often there are several plants of a less than good one.
We do have a row and usually two for the best of the second year seedlings, grown from cuttings. These rows are some of the most attractive in the garden but still there is a high percentage of “almost good” ones and always a few “why did we keep that one?” in there too.
