Should professional Catsitters be left a tip?

I had a Catsitter for years that was a personal friend. I paid her a flat fee and that was that. I have now established a relationship with a professional sitter, due to the health of my friend; she comes in twice a day, each visit costs $15 ($20 each on Holidays). The first time we used the service was over Memorial Day; I didn't leave a tip. She will be back this weekend and I was wondering-am I expected to tip? She does a good job; we don't expect anything out of the ordinary. Just curious...