77-99 in loose passive games

[PokerDork]

One thing that has always troubled me is when to limp and when to raise with the middle pocket pairs 77-99, specifically. Lower pairs seem easier to play as your primary goal is to either flop a set or steal the blinds- although not so common in loose low limit games. Anyway, in what position do 77-99 go from open-limp to open-raise? Typically for me the line is usually MP1 for 88 and 99 and MP2 for 77, although not always, and I try to make the play based on the game conditions. Am I open-raising these too early? Also I typically raise one or two weak limpers with these hands if I'm in the Hijack, CO, or button, and the blinds are reasonably tight. Does this seem right. I don't have PT (because I have a mac) so I don't have any real statistical analysis, but is this about where it should be?