Source: Gary Leff - View from the Wing
More than anything inflight you hope your seatmate actually uses the lavatory when needed. You use the lavatory even when the seat belt sign is on. Just be sure to only use the lavatory in your own cabin. And if you leave it stinky ask the flight attendant for a used pack of coffee grounds.
Chris McGinnis surveyed people about whether it’s best to leave the toilet seat up or down in an airplane lavatory.