The ex factor

I've never kept in touch with any of my exes. It has always been a personal choice of mine since I felt like after spending that much intimate time with another person I felt I couldn't just switch from lovers to friends like one would do switching jobs. I can't imagine hanging out with an ex and reminiscing about stuff because it'll only be about times when we were together so it would seem weird. So when my path crosses with an ex I don't know how to react or behave. Should I say hello? Should I pretend I didn't see him and ignore him?