Depends on a couple things.

First of all, my read on my opponents. When I'm playing online, I rarely have good reads on more than a couple of the other players at the table. This is due to the 12 tabling + no HUD, which many of you subscribe to. If I have too few reads, I'll limp 99 hoping to hit my set or take down a small pot on a friendly board. If my reads on the table are good, then I'll do the opposite and play the pair much more aggressively. Unlike AA, 99 is much more difficult of a hand to play postflop without reads.

Secondly, my decision is based on my image. Keep in mind, my reads trump my image. But if I have solid reads on my opponent + I have been more aggressive than average preflop, I'll raise 99 preflop when folded to in EP. Hell, I'll raise it from any position if no one has raised preflop yet.