This is going to sound really bad, but I completely ignore my final gravity until it comes time to bottle.
I leave my beer on the yeast for a minimum of 4 weeks, except for when I'm dry hopping, in which case I pull it off the yeast after 3 weeks. I figure that, if the terminal gravity isn't what it was supposed to be after a month, there isn't anything I can do to fix it.
Some caveats here. I don't underpitch, I control my fermentation temps, and I give the yeast plenty of time to do their business. I have never had problems with bottle bombs or a stuck fermentation. Also, I'm really thoughtful about the yeast strain that I use and I pitch the appropriate amount of yeast. I'm convinced that most homebrewers chronically underpitch, particularly new brewers.