I generally make it a policy to metagame roughly as much as the players do. If they stay in character, so do I. If they start talking about things like "he won't get another AoO unless he had Combat Reflexes, and almost no giant in the Monster Manual has that", I have this strong urge to give a giant Combat Reflexes as a bonus feat... not an urge I always act on, mind you!
A running joke in my game is that whenever a group of monsters comes along that I've already decided will attack the PC's on sight, one of them says, "roll initiative". That and my huge Treant mini saying, "many of those (insert monster) were my friends" with my best Treebeard voice.
"Many of those fiendish girallons were my friends!"
"Many of those githyanki were my friends!"
"Many of those Psion 3/ Wizard 3/ Cerebremancer 6's were my friends!"
