This is a very common question. And like in many cases, it will depend on various factors.
We must start from the premise that the crew will never expect a tip; however, if given, it will be appreciated.
From there, you should know that it is not mandatory to leave a tip for any crew member. Personalized and discreet service is already guaranteed with the hiring of the service.
However, many clients, satisfied and pleased with the service provided, choose to leave an extra as a sign of additional gratitude, and this is a very personal act.
Ultimately, there is no manual or general rule for what is appropriate in this regard.