First you need to make sure they are not hybrid. If it is a wave petunia there is no way that it will be true. I have collected seed from wave petunias thinking they will at least come close and I only had foliage and never a bloom.

There are heirloom petunias but how far apart in spacing or how to keep them true I have not figured that part out yet. All I can say is try it and see what happens. I mean gardening is trial and error.

Also you may want to post this on the seed saving forum that is if you do not get another response. I know there are several that visit that site frequently that are really good at saving seed.

I sure hope you find an answer cause that seems to be the million dollar question. Some things need to be spaced 1/2 a mile then some things can be hand pollinated and bagged. Maybe that would be an option for you to hand pollinate it then bag it with some kind of breathable material like a cheese cloth.

Here is one of my favorite sites for seed saving I hope this may give you a little help.

Here is a link that might be useful: seed saving