Not really, they flower on regardless

i always deadhead mine so that they last right through until end september,as if not they go to seed.

Really, Helen? I've grown surfinia petunias on my balcony for nigh on ten years, and never deadhead, and last year, one was still in flower when the snow was on the ground (pic in my photos).

I find they do grow quite prolifically irregardless but I do dead head them as it looks nicer. Plus, it lets the plants energies concentrate on the new growth. Hope this helps.