First and foremost, the ring height should fit the shooter and give him a parallax free position. (Your eye should be centered in the optics of the scope). the normal reason to keep the scope as low as possible and achieve the best eye alignment, is to keep the scope optical center as close to the barrel bore centerline and to minimize the error in the point of impact if the rifle is slightly canted.
Bench rest shooters dont have the same problems as position shooters and hunters do with ever changing positions and normally have a level shooting position and/or a built in level for reference so the height is set for other reasons discussed.
Most Long range shooters have levels attached to aid in preventing canting error, even though the scope is mounted as low as possible.
It all boils down to preference and the distance to be used. the longer the shot the more effect canting can have, the higher the scope, the more effect canting can have.
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