Agree that it depends on the situation. If I'm in late and one or two people have limped in I might make a small raise. If anyone has raised before me I'll likely fold (depending on their history). If you're in early position I'll often call the blinds (if it's early in a tournament or I have a good stack in a cash game) or fold (if it's late in a tournament and I have a small stack or it's a cash game and I have a small stack). Other players styles do a lot to determine my play. Also, very occasionally it's fine to present as a higher pair to mix up your play so that it's harder for people to get a read on you.
All of that said, unless you hit your set or you're in late and no one has bet, I'd almost always fold post flop if there are higher cards on the board.