I've seen people pull out notebooks a decent amount of times. I don't see anything wrong with it. I wouldn't keep track of everything. Maybe hands you feel you misplayed, write down a summary of the hand. Definitely keep track of how long you played, how much you were + or -, hourly rate. Don't slow up the game though, and I'd say make it obvious you're writing, cause if you do try to hide it, you'll look suspicious and look like fish. No one should care.