I suppose if a DM were to consider the issue, but not utilize it to take advantage of the PCs, one could consider:
any living space/encampment would have food prep area, sleeping, latrine areas.
People who are not on guard/relaxed with an area probably go to the bathroom more often/casually. They assume they are in a safe place, etc.
People who are on guard/ready for action (like an adventuring party) do not tend to have the bladder of a 3 year old on a car trip. They can hold it for the duration of any action that is about to go down.
When a person wakes up, they most likely to need to go. In fact, indians would drink a bunch of water before going to bed, which would wake them up early (to go on an early morning raid/hunt). A primitive alarm clock.
As a GM, I would certainly gloss over any details of catching somebody with their pants down. that's just gross. Even in a planned ambush, you'll catch him as he was about to drop 'em or as he was just buttoning up.
I have no plans to model the bathroom cycle of the NPCs or PCs. If the PCs are monitoring the bad guys, I suppose I might have an NPC go off to the latrine, in order to introduce a variable the PCs might take action on (because I get bored of describing the NPCs as sitting around the campfire, while I wait for the party to take action).
If the PCs do somehow make it a frequent tactic to attack at the bathroom, they'll get a reputation as FecalFreaks or somethhing. If you make it a practice to not leave a man his dignity, you must have none of your own.