I dont think there are any strict rules as to the height of gravel as long as you have enough to cover the bottom of the tank then that should be fine. From personal experience I have always tried to have about 1 inch of gravel that way any waste get into the stones instead of just lying on the top but I have to mention that most of the tanks I had I used and under gravel filter too!
Gravel plays a roll with cycling, bacteria consuming and personality in your tank. I usually have mine around 2" from the bottom so that it wouldn't consume too much waste caught in between and enough to vaccum out without destroying much bacteria.
But as said.. its really up to the owner on their interest and imagination.