When I was taking the BRC on the 250s, I had to be kept reminded on not covering the front brakes, and when I did use the brakes, to use all four fingers instead of just using the index/middle fingers when braking.
When I was riding my 500 about, I was still using the index/middle on the front levers.
The main times I’ve covered/clenched the front with four fingers was when I was mounting, or sitting at a stop.
To me, I think it’s a matter of personal preference of the rider, and how comfortable their hands are in relation to the lever and finger/hand grip. I could be wrong?