Yes you have to pinch the dead flowers off of any petunia, cascading or not. If you don't the plants will shut down flower production and you will get fewer and fewer flowers until they shut down completely in August. The main purpose of any plant is to make seeds and once it's done this most plants will stop making flowers.

Just behind the flower is a little ball that is the seed pod. To deadhead pinch behind the flower getting the little ball that holds the seeds. It doesn't matter if you go just behind the flower or closer to the stem as long as you get the seed pod

thanks oilpainter I'll continue deadheading.Your info is appreciated

My preference is to pinch off the entire little stem connected to the described seed pod, as I prefer not to have it remain on the plant sticking out.

On the website of the "Wave" petunias they say you do not need to deadhead them. But I do prefer them to look tidy, and enjoy tinkering with them in the morning as you do.