Not sure if there is still valet parking right now - this is something that I would suggest calling Disability Services about (to see if it is still an option at the Resort you will be visiting)
Don't try to call the Resort directly - you will actually be speaking to a centrally located call center, and sadly, those folks are not always current or correct about what it actually going on at the Resorts.
So, Disability Services will be your best bet to actually find out what the situation is currently. You can contact them at at (407) 560-2547 or email
disability.services@disneyparks.com
If valet is still available for folks with a current license plate, hangtag or placard, and If it's a "no contact" kind of thing, where you just hand over the car, I would tip $5 at drop off, and $5 at pick up.
If the CM helps in any way - like they get a wheelchair set up for you, or they do anything other than the basic pick up/drop off, then I would tip at least $10 when you drop off the car, and $10 when you pick up.
That's just me. I know I probably over-tip at WDW, but I am always aware of the fact that those CMs make minimum wage, and are on their feet all day, dealing with a public that is not always as "magical" to them as they are to us.