I joined Rover last year and didn't keep track of expenses since I figured I wouldn't have nearly enough to make it worthwhile (itemized deductions vs. standard deduction). But then someone recommended that I keep track, so for 2016 I have.
The way I figure it, you would include mileage you drive while caring for the dog (transport to and from the dog park, or a trip to the store to buy dog food if the owner runs out), but not the mileage to and from the owner's house (since a person with a "regular" job wouldn't count the mileage to and from their place of business).
There's also the cost of business cards and other advertising/promotional material you may use. Also, sitting-related supplies you buy exclusively for your use with client pets, or stuff bought for client pets.
I don't know if this will do me any good, but I figured it doesn't hurt to be prepared.
Good question! I was wondering the same thing....