Hi, welcome to GoY, a lot depends on what group of petunia you're growing, they're divided into 2 groups, grandiflora, and multiflora, the grandiflora types usually have larger flowers, but the multiflora usually are naturally more bushy, personally I wouldn't pinch the tips out of either, there are about 40 species and countless cultivars, but all are in 1 or other of the 2 groups, but you wouldn't harm the plants if you did pinch them out, hope this helps, Derek.

Definately not, they're self branching and will bush out nicely on their own.