what percentage should you use to tip the pet sitter?

Honestly, as a sitter myself I don't think that you should look at it as a percentage. I have had tips given to me from $10 to $50, (even had one $100 during Christmas time). As much as a tip is always appreciated, I don't ever expect a tip or have a negative feeling about the clients that don't leave cash tip at the end. What I really do appreciate is knowing that the clients are happy. (And yes, I honestly mean that!) The best tip that I can get from a client is when the refer their family and friends to me. When they leave an awesome review (any review is always great but what I love is when the clients mention what it was that I did that they appreciated the most.) I truly appreciate the feed back and it lets me know that the client is happy. I try to go above and beyond for all of my clients. So to me when I see that they noticed and appreciated what I do, that honestly is the best "tip" that they can give me.
So as much as money is always great. I would never want a client to feel like they need to or have to leave me a tip, let alone a certain percentage. A review and referrals is the best way a client can let me know that I am doing an awesome job and that they are happy with my services.

Tips are not required by sitters but they are very appreciated. I consider my service as a sitter to be on par with other personal service providers. If you feel the sitter provided excellent service, then by all means give them a tip (or gift or other expression of appreciation). The best tip, however, is a great review and referrals to your family and friends.