I agree with the Doctor. The number of limpers and position certainly should be taken into consideration. With a lot of limpers in front of you, limping along is the best play, adding to the number of players seeing the flop and possibly hitting two pair or whatever against your set if you hit one.

Another bonus is that if you just limp in and someone behind you raises with a big pair, they're probably still pricing you in, laying you all kinds of implied odds, especially if some of the limpers come in too. If you raise, of course then they'll be more likely to make a huge reraise with a monster hand, not allowing you to see a flop and throwing away what you put in the pot.