There is no waiver for sitting height being too high, only for your weight. It's for if you're on the lighter side (ejection force may screw up your back) or if you're on the heavier side (the chance the ejection seat might not get you high enough to clear the tail or for the chute to deploy if ejecting on the ground). It's not guaranteed something bad will happen to you, the Navy just does it to clear them of anything because the chance exists.
As for height, if you're too tall you'll break your neck regardless as it'll be your head breaking the canopy, not the seat rails.