What I've noticed is there are a couple different schools of though on this mid to lower pocket pairs. Some people think that the best way to play them is to play them from mid to late position and try to get in as cheaply as possible and hope you hit your set on the flop. If you are in the higher range (7's or 6's) you may continue on if the flop is dry and you picked up a couple outs. The biggest draw back I've seen from this is the fact that you got in cheap. If nobody raised preflop and you just limped in, they probably don't have anything they will stack off with now. When you do hit your set you are really looking to stack off. This method you can still hit your set and no one will pay you anything.
Another side is you'll want to still play them from mid to late position, however, this time you'd rather play them when it's been raised ahead of you. If you hit your set now, you are more likely to be facing someone who has a decent had and may stack off. The major problem with this is effective stack size. The raise you call can't be so big that it takes away the profit you are going to make. If the bet ahead of you is $300 with your opponent only having $1000 left, you don't have the right implied odds
I kind go somewhere in the middle. It really depends on the type of game, stack sizes, and any reads I have on whom ever I'm playing against. I will say, some of my largest pots have been from hitting a well concealed set on the flop and stacking off. I have also lost to set over set though too, so take it for what you want.