I am a dog walker and no, a tip is not customary.
That being said, plenty of people have tipped me but more often for pet sitting if they're going away. NOT a daily. Some of those tips are quite generous. It still is never expected and shocks me every time haha.
Of course, even though my clients know I'd do alot for them with NO TIP (like quick freebee visits if I have a concern about the pet or stay a long time if the dog is upset with fireworks or whatever) it also is a motivator to make sure I do extra for those particular people so it's a win-win I guess.
When I had a dog walker years ago I would tip BIG for Xmas. A weeks pay PLUS the week off with pay(because I always took off so that equaled TWO weeks pay) and a decent gift.
I also always paid her for days that I cancelled even if her policy was to let the client slide.
I am on sites with hundreds of pet sitters and the topic of tips never comes up. It's not expected. Sometimes we chat about gifts or if someone gets something really great LOL.
The other time people tip and it's appreciated is when they have all goofy schedules and constant changes. Whenever someone insists on paying for a cancelled job or whatever is very nice. I had a guy hire me and spend 3 hours of my time then cancel the trip. When I returned his garage remote he paid me the cost of the three hours that he estimated he tied me up without it turning into a job. (the Meet & Greet, phone time and driving time). THAT is pretty rare and when he decides to go away again, I'll won't hesitate to book him. Other times if a new person is too chaotic we aren't that willing to trust them the next time...it all depends.