I'll sometimes become a bit "competitive" if someone passes me on a hill at a reasonable clip while I'm going at a relaxed tempo. Why not? The 6% grade on the way to work is fun to try to go up maintaining double digit speeds. I definitely keep my distance (either behind, to the side, or in front) and keep things safe. Just purely friendly pushing of each other to try a bit harder. If I can't pass with a reasonable speed differential (to minimize the time side by side), though, I'll stick back and let the other person go, perhaps trying to catch up at the top. If someone wants to suck my wheel, that's cool, as long as that person maintains a safe distance. It doesn't bother me the least bit. Overlapping wheels, on the other hand, is not cool.