I open all pp's for a raise from all positions at 10nl 6max and FR. I sometimes will limp small pp's (but always raise 77+) from EP at 25nl, where you get 3bet more often. Several FTR discussions have been held on the topic, many of them in the Beginner's Forum, over the past year or so. Basic consensus is that you should open raise all pp's from all positions until game conditions force you not to - meaning, there's too much 3betting behind you that forces you to fold preflop since you rarely have set odds to call the 3bet with 100bb stacks.