You know the situation. You need something specific: a land, say, or a removal spell, so you play Ponder. You find the thing you're looking for. Hooray! Unfortunately, the other two cards that Ponder shows you are completely useless at this point in the game, so if you put the card you want on top and draw it, you're guaranteeing yourself two dead draws afterwards. On the other hand, if you shuffle, you might have better draws on the upcoming turns, but you leaving it up to randomness whether you draw the card you need right now.

What do you do? What factors do you take into account in order to decide the correct play?