Limp calling preflop basically turns your hand face up and you have to fold when you miss your set because limp calling preflop with pocket pairs is specifically a set mining strategy. If you start limp calling then calling multiple barrels, you're doing it wrong.
I prefer to make a standard raise with all PP's from any position. This means very often when people flat (even in position) you have the initiative post flop, and they will often have to fold to your c-bet. It also disguises your hand significantly.
The only problem with raising all pairs is that you will have to be willing to fold to 3-bets. Most people arent 3-betting much worse than AK at micros and they certainly arent 3-betting small pairs. Also, depending on effective stack sizes, sometimes flatting the 3-bet for set value can be profitable too.
Also, I think when you get 3-bet you can call a bit lighter than the "Call 20" rule since peoples 3-bet range is so strong they will stack off more often post flop. Certainly with overpairs on an 9 6 3 board when you have 66.
Thats my opinion anyway.
Edit: Some players can easily get away from overpairs when people limp-call preflop because a set is so obvious. It's a lot tougher for them to fold an overpair in a 3-bet pot.