Not all dahlia varieties need topping, only the larger varieties, when it should be done depends on how you want the plant to grow, if you want to grow a single then you remove the remaining sprouts as they emerge from the soil. When it´s time to top ? when the plant has grown tall enough but not yet hollow inside ( around 10-12" ) that´s when you remove the top, the process can be repeated again on the branches for a bushier plant.

Pinching the buds does produce larger blooms, significantly is a matter of interpretation because depends a lot on the variety, is some varieties ( like dinner plates ) it is really significant, in other varieties is not that significant. One stem has three buds, a main one in the center ( the largest ) and two secondary buds, one to each side of the main, you pinch the secondary buds as soon as you can spot them, that means as close as you can to the main buds stem.

Topping is done to make a fuller plant on any sized dahlia. It will also keep some of the taller ones a bit shorter.
You want to disbud not just for showing but so you get a long enough stem to have a nice cut flower-- not a bloom on a 3" stem.
Ellie