Go to the Colorado Dahlia Society Site & look at the care & culture part- there's pictures & it explains everything you ever wanted to know about growing dahlias.
You will be pinching out the growing tip of the plant so you get more laterals with more flowers on them & a bushier plant.

Here is a link that might be useful: Colorado Dahlia Society Site

I only pinch when the plant is small, and the stems are not hollow. Once they get larger, I do not pinch. I do dead head though. Mine are blooming well now.

I am trying something new this year, that I've read as a recommendation many times before. I didn't pinch this year, but I am dis-budding.

Typically, when you see one bud on a branch, if you look real close you'll see 3 of them. Remove all but the center one. This is supposed to make for larger flowers. I also believe they'll be healthier flowers.

You'll find references to it on the sites mentioned, I beleive.

Cheers,
Russ