15-18 is max for me also. Granted, going higher may help in the scent and being seen scenerio. However, the benefits of a good shot angle outweigh them to me anyway. In other words, I'd rather be busted then not have a good shot angle. Good cover is essential. Keep in mind that I won't shoot past 25 yards.
Your choice of tackle probably plays into this, as well. I won't go above 15' for my first year really shooting traditional gear. The angles (at a higher point) would make me REALLY nervous. I'm not that good.