I was assuming that pet sitters in this context were neighbours who popped in to feed the cat or collect the dog to take for a walk when the owners were out. House sitting is very different, and of course a house sitter would have to use more than one room. Spoaching around bedrooms other than the one they are using would be beyond the pale for me in that situation, but obviously they would need to use the kitchen, bathroom and sitting room, as well as the room they are sleeping in. It's not a service I can imagine using, really - far too stressful 
A friend or neighbour popping in to feed or walk animals shouldn't need to go upstairs at all, unless the only loo is up there and they are caught short. My children both use professional dog-walking services, and that was all discussed when they came to discuss terms. They have keys and let themselves in, call for the dog, take him out for the agreed length of time, and bring him home again. They go in the kitchen to make sure the dog has enough water left, and then leave. Neither of my children have cameras other than ring doorbells that show arrival and departure if they check. They wouldn't give someone a key to their homes and access to their dogs if they mistrusted them enough to feel that necessary.