When is stealing PF in a blind appropriate?

General question. What minimum conditions should be met as far as stealing preflop in the small or big blind at a full table?

Small, avg, or large stack?
# of limpers?
Minimum hand requirements?
When you have a strong hand and a raise has been made in front of you?
Tight/loose table?

Again, very general question. I feel like everytime I make a move in a blind people think exactly that, that I'm making a move.