I pinched the center leader out of my tall dahlias as I read to do. Now they have branched and I don't know if they will still get 4 feet tall as I wanted?
I find that they don't get quite as tall after pinching, but they are fuller and give more flowers. However, mine grew to close to 4 feet last year after pinching. If one wanted to "show" your dahlias in a competition, then one would be better not to pinch, getting larger flowers and straight stems. If one wants more flowers, pinch the leader and get more flowers, albeit a little smaller flowers.