Berandor is right, and so is Tsyr. 
In my game, I gave the players a list of "common sense": common creatures that everyone knows about. Troll would be on that list, as would ogre, giant, and another 25 common enemies. Anything not on that list is fair game for modification, so ogre mages might not be green and look the same... they might be their own monster entirely.
In this way, you can have some comfort levels and some surprise, and that's what my players like the best.
I wanted to throw in another phobia:
As a DM, it freaks me out to see the players flipping through the MM during the game. The first time that it happened, we were in the middle of combat, and I thought that the player was looking at the enemy's stats. So I reacted with a hothead, and it turned out he was looking for a monster to summon.
Does this happen to or bother any of you?

In my game, I gave the players a list of "common sense": common creatures that everyone knows about. Troll would be on that list, as would ogre, giant, and another 25 common enemies. Anything not on that list is fair game for modification, so ogre mages might not be green and look the same... they might be their own monster entirely.

I wanted to throw in another phobia:
As a DM, it freaks me out to see the players flipping through the MM during the game. The first time that it happened, we were in the middle of combat, and I thought that the player was looking at the enemy's stats. So I reacted with a hothead, and it turned out he was looking for a monster to summon.
Does this happen to or bother any of you?