Slightly off topic, but related, my current, long term ( 15 years) was really concerned about his exes. He was never going back there ( there were three of them) but I have met each of them which, 1) made my husband a lot more relaxed and 2) gave me some real insight into his "type". So back to the a would he date any of them? 1) over my dead body, 2) if I am a dead body highly unlikely!!! Talk about recognizing and enjoying the past but happy to leave it there, he just isn't that bloke any more.
So date an ex if it is recent enough to pick things up happily where they left off, otherwise, leave the past to history, move on and enjoy!