This is a simple concept: If you use the last of a toilet paper roll, replace it. There are few things worse than rushing to the bathroom in an emergency only to find a bare paper dispenser. There are karmic forces at work in the bathroom etiquette world. What goes around comes around. Think of it as protecting yourself in the event of future emergency.
And while you're at it, you may as well put the TP in the dispenser the right way. Yes, there is one. The loose end of the roll should lie away from the wall -- the so-called "waterfall method" -- so as to be easily accessible. It's especially helpful for those "hoverers" who have the delicate task of ripping the paper while balancing themselves precariously above the loo [sources: Fennessy, Ryan].