Here's what I think - the average person doesn't really know much about PTSD. So if you tell someone you have PTSD from a traumatic event(s), they may shrug and say whatever. Or not really know what that means.
It's not like schizophrenia that most people know is fairly serious and might research right away if they're considering getting serious with you.
I think if a person knows they have PTSD and are knowledgeable about what that means and the problems that could arise in a relationship, then they should share that aspect too. And in that case, I personally would wait to tell someone until I know for sure that I am interested in pursuing something more serious with them.
Unfortunately, I think a lot of times even if someone knows they have PTSD, they don't quite know how it's going to show up in a relationship (this is my current experience with my guy), so I think if you don't know how you personally as a sufferer will be affected or show up in a serious relationship, you should research all of this as much as you can before diving in to anything serious.
My two cents!