SALVIA AND PETUNIAS. Brand new plants. Just put them in the dirt - should I cut the tops off them?

I have heard this makes them bush out and makes a sturdier plant with lots more flowers. Also 'pinching back' for the petunias. I don't mind doing it to the petunias, but the Salvia only has that one flower on top. To cut or not to cut - that is the question.