For my own band's music I always quad track. 100% L/R and 80% L/R. The 80% tracks are -3db lower in the mix. I don't quit tracking til they're tight as fuck!
For clients, however, depending on what their playing is like, what their music is like, and what the time constraints are, I will do a mix of quad and double tracking. I may, for example, double track a song for the most part, but throw in two more tracks on bits that call for a beefier sound. I automate the rhythm guitar bus to drop 1-2db when the other two tracks come in, so the guitars don't become "louder", just "thicker".