Very important question, that needs a very long answer.
As I consider limping to be a very bad thing to do in any MTT, it leaves the options to Fold or Bet pre-flop.
From early positions (UTG - UTG+2) I would say that folding smaller pocket pairs is fine. I draw my line between 77 and 88, so basically 77 is a fold, 88 is a bet pre-flop.
Mid positions, pretty much the same thing, only I tend to add 66 and 77 to my betting range.
Late position, especially BU (Button) I tend to bet with any pocket pair, even deuces.
This is just a rough explanation, but it depends a lot on my stack size (sometimes I will fold more, while sometimes I would shove any pocket pair from any position).