I'm completely flat, and it was done on purpose. I purposefully did not work out my chest at all before surgery, I requested that my surgeon take as much tissue as possible to make me completely flat, and I have done very minimal work on my chest since. I do not want anything that could ever resemble or be confused as breasts on my body. I don't care how buff or toned the rest of me gets, that is the one area I don't work out at all.
If you work out your chest after top surgery, you will get pecs. They aren't removing the muscle. If you have developed your pecs prior to top surgery, you will come out of top surgery with visible pecs most likely. If you want pecs, it is a good idea to go in with them developed so that the surgeon can sculpt you easier, if that makes sense. But it doesn't make too big of a difference IMO unless you are totally jacked.