Most were decided by a skilled paddler who likes big water (and when boats were big). Most of the lower ends have been adjusted for the smaller boats. Whether high water is the best time to paddles is up to what you like. Most people like enough water that they are not scraping rocks, but not so much that things get out of control.
Low water is technical, lot's of maneuvering, high water gets pushy with much less maneuvering. Swimming in high water can be really scary.
Start with yellows and migrate to green as you see fit.