I have to use the lav a lot and always book an aisle seat. Sometimes only windows are available though when I clear the upgrade list or if I do a Same Day confirm.
I figure if only windows are available the person in the aisle already booked it and don't have an issue waking them up if I have to go. If they don't want waken up they should have booked a window seat. I don't have an issue being woken up if the person in the window has to go since I booked the aisle and figure that is the price that comes with an aisle seat.
If someone has an active bladder they should book an aisle seat if it's available when booking or being upgraded.