SteveM smaller flowers and skinnier stems would seem to be NOT a good thing, especially late in the season. I had never done it; did not see this thread, and started a thread about it, due to what I read in Kristine Albrecht's dahlia book - that it was supposed to help prevent hollow stems, which excited me in this very wet fall/winter climate. I have had a lot of trouble with rot in clumps happening fairly fast, due to hollow stems.
So I did this as an experiment with some of my tall varieties and a few A sizes and B sizes. Now I regret messing with them!
I wish I had read this thread first. Ted tried to steer me right in my thread...but I was a dummy and tried it on some anyway - I won't make this same dumb mistake again since my dahlias are already so late to break dormancy and many are less than 6" tall. I'll be lucky to get a month of flowers... weather permitting. (slaps head)