Depends
There are many factors involved, type of tourney, position, position, most of the time I would say wait for the flop. If another player has decided to go all in and has you covered, you are the only one taking a bad risk, they stay alive even if you win, but you are gone if you lose.
If I am going to risk my tourney life it will be on a hand that has better chances then a middle pair all things considered, unless I flop a set, then All In I go. Most of time at least, of course I have been known to fold even with a set if the board is wet and has over cards paired.