An indy has to provide an incall or pay for/arrange transportation to an outcall. Either of these is an expense for her.
Incall - If it's a first encounter, I usually usually leave a tip if booking was a breeze, and if I really enjoy the company and the service. Subsequent incalls - a tip included with my initial payment, or a modest gift if I know there is something she enjoys.
Outcall - Depends of whether she charges an additional fee (for transportation) for the outcall. If so, I tend not to tip unless she voluntarily stays longer than expected or the service is exceptional. On subsequent visits, a tip and/or a gift.
Gifts....typically pet treats if she has a pet(s), chocolate, gourmet pastries, high quality coffee, flowers (orchids).
...but then I've been told that I have 'excellent reviews'.