The very best providers, in my experience, do not ask for any money upfront. But I am okay with that if they have some history, many reviews, and all good ratings.
Related, but if I pay up front, there will generally be no tip. If I pay afterward and I had a decent time, there will be a tip. I don't think I've ever paid in advance and then given a tip after the fact, but I suppose it could happen if it was really enjoyable.