Definitely 99 and 88 are in that grey area for me. I actually have raised pre-flop w/ 99 in the past (not 88 though).

I play all pocket pairs -- 22 and 33 are not rags!! I hope to limp in and hit the set or better (11.8% chance of doing that on the flop).

If not, I usually just them go to any big bet.

The implied odds absolutely justify playing any pocket pair in any position (but not against raises) - this is one of the key differences in my strategy between limit and no limit.

I may try raising it up with 99 on a weak table, but I think 88 I'll still look to limp in.