These are interesting hands - some of the most profitable and some of the hardest to play... i agree with what's been posted, although i think you should also keep in mind the whole 'pump it or dump it' thing if you're first in.

I could be wrong and I need to assess it for long-term profitability - it *could* be a leak! - but if i'm in MP and i have a pair of 5's (for example) and everyone's folded to me, i'm raising it up. (depending on the table 'feel' of course)

If everyone's folded to you and your hand isn't worth a raise, why would you play it? Post-flop bets have far more credibility as well.