There's no easy answer to this question - it depends on a number of factors, including what the dealer is showing, what your hand looks like, and whether you're playing for fun or trying to win money.
In general, though, hitting on a 12 is a risky proposition. If the dealer is showing a low card, you might be tempted to think that they're likely to bust - but remember that the dealer always has an advantage in blackjack. They don't have to take chances like you do, so even if they are showing a low card there's still a good chance they'll end up with a strong hand.
If your hand is weak then hitting on a 12 is probably your best option - but if you've got a decent chance of winning by standing pat then it's probably wiser to play it safe. In the end, it's up to you to weigh the risks and rewards and make the decision that feels right for the situation at hand.