I will say, I don't mind occasionally checking top pair in heads up play, especially against a known aggressive opponent who loves to bluff. I would never take it farther than the turn though. If you haven't induced a bet by then, you need to take the pot down quickly. In any pot with multiple players, or basically any pot in a full ring game - regardless of how many saw the flop, be it two or ten - I'm betting that top pair.