I think it depends on the type of petsitter that you have.

In our case, we hired and pay the girl directly. She keeps all the money as if it were a babysitting situation. We pay her well (more than her other 'gigs' but still less than it would cost us to use a kennel) and we didn't tip extra. I assume that if we continue to use her and we go somewhere around the holidays, then a holiday 'bonus' would be in order.

When we lived in NC and we hired a service to pet sit (and we asked that pet sitter to do more than typical with medications and walking a dog-aggressive dog), we tipped. We figured that the regular fee paid both the sitter and the company she worked for...the tip was hers to keep.