Depends how often you want to wake up. I usually go 190 on the downside. The meat should still be cooking nicely at that temp. If it does drop, I can get it back up fairly quick by stirring the coals.
On the high end, I want it a little tighter, about 260. If it's spiking for some reason, may be harder to get it down if you let it go too much.
The bottom line is, if you've gotten it stable before going to bed, and there aren't any freak weather happenings, the alarm shouldn't go off. If you're like me, you'll wake up a couple of times and check it anyway... /infopop/emoticons/icon_wink.gif