Hi Jackie

best to nip it out as it will make it more bushy and you will end up with more flowers ....

Unless you want giant blooms and then you take out all the side shoots. My dad had some magnificent blooms.

If they are about to bloom, it is too late to pinch--all that you will accomplish is to remove the first flowers. Better to pinch them much earlier, when the sprouts only have two or three pairs of leaves.

Topping off the dahlias will result in a bushier, sturdier plant with more flowers than if you didn't top off. However, deadheading all the spent blooms could be tedious.