Always the person who used the last sheet because the next person up may not notice it being empty and start their business and not be able to get a new roll. Depending on where the fresh rolls are kept.
@fun4us2b yes, any toilet. Most people use multiple sheets per visit, so the last piece on a roll is much more likely to be one in the middle of a session than the last.
Obviously, it's the person who uses the last sheet. Furthermore, if they don't change it, other people in the house have the right to hide the toilet paper from them until they do.