It's up to you what you want to require from clients, but I would definitely at least ask whether their pup is up to date on vaccinations. Even if you only host one client at a time, an unvaccinated dog is still at risk of infection if it comes into contact with contagions on a walk (anything from other dogs to a contaminated puddle of water). I also make sure every dog that stays with me is on active flea treatment because ending up with fleas in your home becomes a huge hassle to deal with.