This is a very simple principle: If you use up the toilet paper, you replace it, not just lay it on top of the toilet tank. Hosts should be sure there's an extra roll within reach, especially if they don't want their guests digging around under the sink and looking from something to wipe with in a pinch. Those frilly towels that Aunt Ginny gave you as a housewarming gift were meant to be used on hands.
The trick here isn't just setting up a fresh roll when the last one runs out, but setting it up properly. Use the "waterfall method" to ensure that the loose end of the roll is facing away from, rather than up against, the wall. This allows users to access the TP with a gentle flick of the wrist, instead of having to reach and scoop. This is also the practice hotels use, as the paper flows more freely and looks attractive with that little decal pressed into it. But some people think the underhand method actually makes it easier to tear the paper off the roll and offers less wastage.