It really depends on payout structure. In a flat payout structure (1st place gets a small % of the total prize pool) start stealing earlier. You should be stealing when the # of players remaining is about 1.5 times the places paid (obviously, this is a general rule, don't follow exactly). Playing a strong, aggressive style with the chip lead is quite clearly the way to reach the top levels of the payout structure (which should be your goal).
I have been known to raise large preflop 90% of hands when I am in the top few stacks in a flat payout structure. You often will coast to the final table and far into it, where the
real money is. Even in a flat payout structure, 1st is a huge payout in any tourney of decent size. It is usually better to finish 1st one time than 10th place 5 or even 10 times.