I make a point of leaving a bit of "headroom" on the master. My approach is as unscientific as it gets. As far as how many dB to leave, or how much -dBv or +dBu or whatever, I have no idea.

I simply look at the meter on the master channel and go beyond "making sure it doesn't go in the red." I used to think it was okay to let the master meter "tickle the red," but if you're doing a quick and dirty mastering job, you should leave more headroom than that.
What I do is keep an eye on the master meter the whole time I'm making a song. I try to keep its peak well below the red. I try to keep the master meter 3/4 of the way up. I keep it a good few segments below where it would be "tickling the red."
I've read lots of threads on this board over the years about the mixing process. There was a good one once where "going into the red" came up. The basic gist of the thread was that it is okay to go into the red on regular tracks, but the master should never go in the red. So, if you're going for a certain sound that requires driving a track into the red, that's fine, as long as you don't let the master overload into the red. It can be tricky. I'm not sure what the official word is on this, I don't think an official word exists, but, whether it's a good or bad practice to move the master's fader in order to keep it from going into the red, I don't know. But, I do move the master's slider down in order to retain my headroom on the master meter.
Maybe someday I should sit down and read Tarekith's mastering article. It's hard to do when you've got so many LOLCATS to look at, and things like that to do.