If you are concerned about your car, deploy the bulk of the tip when you arrive, to influence the level of care your car is about to receive. And I would also document the condition of the car on arrival. One way to do this is with the video feature of your smartphone. Shoot the odometer. Capture the valet's face and name tag. Then video tape everything with the outside of the car, paying particular attention to each wheel, underneath the front air dam, and each corner of the car, along with the sides.
Tell the valet "Just so you know, it's recording," while pointing to the car.
No further explanation needed. They will soon begin to think you have a video surveillance system.
When you get the car back, do a vehicle damage audit immediately. Do not drive away. Check the car entirely for new damage, and take it seriously. (Once you pull away, it becomes much more difficult to get compensation for damage.)
If everything is okay, only then do you present a departure tip - as a courtesy. And a smaller amount is okay at this step.
If you want more tips on this, just look me up online.