I have this really bad - not just a twitch or a little jump sometimes - an honest to goodness kick. (it's a symptom of the larger leg problem)
Anyway, I always give my tattooer a heads up about it and every one of them has reassured me that they can handle it, that pretty much everybody twitches a little bit here and there, and it's not a big deal.
And so far (knock wood) I haven't jerked at just the wrong moment and screwed things up! (but I do still worry about that *one* time that I end up with some crazy line jutting out from an eyeball or something!!)