My dilemma, as always, is whether to speak or not to speak. I hate ignoring people I obviously know - it feels so junior high. I always feel like the polite thing to do is to at least say 'Hi.' I don't have to carry on a fakey conversation or anything like that, but 'Hi' is something I can get on board with. Then I know that I've at least done my best and tried to be polite and grown-up about things.
Of course, that rule only applies when the other person has definitely seen me. I'm not a glutton.