The last time I PC'ed, it was a bard with a number of knowledge skills, and while I didn't look up monster stats during a fight, the DM was fairly decent with me using what I knew off the top of my head in-game, though there were the occasional bardic lore checks and the like.
There was also a mid-high level game I'd been in a while back where I kept identifying beasts from the slightest of descriptions, and the DM at least didn't look annoyed that I kept doing it...heh. As the game had started at level 13, there was some amount of assumption as to being familiar with various beasties.
Again, though, that was just using knowledge I had off the top of my head.
For my part, as a DM, I don't mind some amount of foreknowledge on the PC's behalf. However, I wouldn't let them pop open the Monster Manual during a fight, or before if they knew it was going to happen. Afterwards, that's fine - just because I've had disagreements with DM's in the past about how a monster worked, and wanted to check afterwards, and I'd be hypocritical if I didn't let the same happen with my players. But that's after the fact.