It's a very simple thing. Common sense.
Before takeoff, use the lav. During takeoff, no lav. During cruise flight, use the lav. If M/Cl lavs are inop, use C/Cl lavs, and if necessary, F/Cl lavs, and vice versa. During landing, no lav. After landing, use lavs.
Diarrhea, need to vomit (providing you can still make it), small children needing to go etc., I will pull my cart out of the cabin, or point you to the nearest lav, never mind what class.
Simple, isn't it?
Yes, we do discourage "lavatory tourism". But there's a limit.
And what about F/Cl lavs being inop, or just occupied? Does this only go back to front, or in the other direction as well?
If you bump me, or even trip me on the way, I expect you to come to me on the way back and apologize. Just as I will if I bump into you while I work.