From my limited experience, when quad tracking, I always have 2 tracks 100% L/R, but for the other two, it really depends how different they sound to the first pair.
I prefer having the second pair of tracks different sounding (minimum different mic placement, but I'll go for a different amp and perhaps even different cab). Then I pan them between 90% and 95% L/R.
I feel that if the second pair sounds too similar to the first pair, it gets over-crowded. And whereever I try to pan the second pair, I'm never fully satisfied, until I turn their volume significantly down... and obviously you start wondering why you use 4 tracks.
But, as you probably know, to have 4 tracks, you either need to edit them or have a very freaking tight guitar player. And if you're in a band with 2 guitarists playing the same riff, don't even think about having both record 2 tracks each! It' just physically impossible to have them both play the tracks the same way. I have tried it and it's just impossible. The tough part is convincing the second guitarist that he's not going to record most of the song. Most of them will not understand that.