I think that tipping is a wonderful way to tell your sitter that you appreciate their work, especially if they went above and beyond for you or your dog. If you were a last minute reservation, or your dog destroyed your sitters home, or that your sitter just did everything you hoped for and then some, tipping is a very obvious sign that you appreciated all they did for you. It's not mandatory, of course, but if you're planning to use that sitter again it's a good way to spread some goodwill and make your sitter want to work with you again, in spite of a destructive dog or something you left out at the meet and greet that ended up being a very important thing to know. Good rule of thumb, you need to remember that this is a part of the service industry, and just like you tip well at a restaurant you go to often or whatever, tipping your sitter is an easy way to show your gratitude and appreciation for all they've done for you. It truly makes our day.