This is actually a pretty good question. I guess the answer to this is that it can be a bit individual, a matter of preference, comfort and what kind of handle you use. Some will tell you to curl all fingers beneath the racket, others will keep two fingers in contact with the handle. But I guess even those that curl all the fingers, usually keep at least one finger, like the middle finger in contact with the handle.
A good example is Dan and Tom from tabletennisdailyacademy.com. Dan explains the grip in the backspin serve video, but you will notice that he curls all of his fingers and has his curled middle finger in contact with the handle. However, Tom keeps his middle and ring finger in contact with the handle.
I did a change in my service grip at one point in time. I noticed that just curling all the fingers under with less handle contact, resulted in a slight drop of the racket head, resulting in more sidespin when I wanted more backspin. So in my setup two fingers on the handle give me more stability and snap. This is not always easy to notice, many people have a slight drop unless they watch a video of themselves.