JJ/TT hold up pretty well heads up. So it's good to raise and make the pot larger while limiting the field. That said, they also play well in a multiway pot if you can get your set. So if many people have already entered the pot (making them more likely to call your raise and stay in), it might be best to limp until you see if you get your set and/or you know that the flop doesn't give someone a higher pair.

- Jeffrey

(That's basically regergitated Sklansky, btw. Take a look at Small Stakes Hold'em and Hold'em Poker for Advanced Players)