The aim of raising small pairs is to gain more value from your sets, while supporting the extra cost of raising by the times you take it preflop or on the flop with a c-bet. If you can't take it down by the flop without a hand, then you're sitting on a goldmine anyway, so just limp it and raise only your premiums.

It all depends on how loose preflop and how weak postflop your opponents are. If you find when you raise AK from UTG that you often just pick up the blinds, or that you often take it with a c-bet on the flop, then it's time to start raising 22 UTG. If when raising AK UTG you always get callers and your flop c-bet is always called, limp 22.

The same idea when you're on the button, except you have more information about who is going to be in the hand, so you can use them to steal blinds or to attack the one weak limper.