Do you insist on up-to-date licenses and vaccines before walking a dog?

I have a question on my pre-stay survey that forces clients to agree that they understand that their dog will not be brought off their property unless they are up-to-date on their license and rabies vaccine. Is this a reasonable requirement? I want to make sure that if an authority checks their tags or if a dog I'm walking is attacked or something, that I'm not the one getting fined or held liable for them not being licensed/vaccinated. (I've actually been stopped by animal control once before and asked if my puppy was licensed and vaccinated.) At the same time, I don't want to deter people just because, say, they forgot to update something. I'm curious what other sitters do if the dog you're supposed to be walking is not licensed or up-to-date on vaccines.