Assuming that you are asking about the NRA or CMP High Power Rifle Competition shooting positions, I have never seen a match competitor fire from the kneeling position when he/she had the option of firing from sitting. We have some matches, usually Garand matches, where you are required to fire from the kneeling position, and it is not nearly as stable as the sitting position. Perhaps a 3-position or 4-position rimfire or airgun rifle competitor could offer a more stable way of dealing with kneeling, but even they will agree that sitting is more stable.
