There seems to be a consensus on this board that most people drop a few points on the day of the test, due to testing anxiety, distractions in the room, etc. You can do certain things to minimize this possibility, such as scouting out your test site in advance and taking practice tests in noisy enviornments. But you can't control for everything.
I think it's better to expect that you'll go down a bit from your practice average. That way, if you do better than you expected, you'll be pleasantly surprised.