I used to be a nanny, and now frequently pet/house sit when my former families go on vacation. They've all always left the money with any written instructions at the beginning of the trip. As for tips, it's usually rounded up to the nearest $20.

New poster. I have another question. Do you pay tip to pet sitter?

I paid in advance once, to an individual. Came back to a bed covered in catshit and half of the food I'd put out for her to administer unfed. Petsitter had been underfeeding, or not coming at all for all I know, and animals had used my bed as litterbox in frustration. Ever since then I use a service and only pay after I return, and all is well.

My pet sitter requires full payment at the beginning, and I leave a tip too (in cash).

I pet-sit. Some people leave me all the money before they leave, and one person does half at the beginning and half at the end.

I always leave payment on the table with a note for the pet sitter.

We have a dog sitter coming to stay in our home for a week. Do we pay when they first come or after the job is over? We won't actually see the sitter on the last day because they will leave before we get home...