When I first starting using a pet sitter, it didn't occur to me to tip. Somewhere along the way, I started doing it. My sitter's visits for two cats (well, now we're down to one, but I haven't used the sitter since then) cost $15, and I tip her $3 on top of that. So, 20 percent. For as much peace of mind as I get from knowing she cares for my pets, I figure it's a worthy expense.