Mauricio, I'm sure that people will tell you that they collected seed from their Wave petunias, and the resulting progeny looked "just like" the originals. BUT, I will tell you that I have seen the parent plants of Waves, and they are nothing like their hybrid children. If you want to guarantee the habit and color, then you need to buy the seed. I'm sure that you will get some interesting, and in some cases, lovely petunias, but they will not all have the same habit.

I allow a few self sown "Wave" seedlings to grow every year and my personal observation is that habit is sometimes quite similar to the parent plant; color is the aspect that is least predictable.
Attenuated leaves(compared to a traditional petunia)are a good indication that the seedling will exhibit the characteristic spreading habit.
If you are concerned about uniformity, purchase seeds.

I let my wave petunias self seed too. I've gotten different colors, but all have been spreading.

Kat

I sowed some seeds and now I have 20 seedlings growing.. I will post pictures of them when they get bigger, and when they start to flower so people can use as reference.

Mauricio