I generally walk back as well, but on some aircraft, like CRJ900s or A319/A320s, there are extended periods of time where someone siting near the front of Y doesn't have access to the Y lavs due to drink service. In that instance it isn't really an option. If you gotta go, you gotta walk up front.

What I've seen most commonly, and what I think is a perfectly acceptable solution, is for FAs on those aircraft to prevent people in Y from hanging out and waiting for the F bathroom to open up (both to prevent a crowd and so F passengers don't have to wait in line behind Y passengers) but otherwise not worry about it. In planes that have a Y lavatory between the cabins, like a 757, I think they can be more strict about making Y passengers stay in Y.