I need my oar handles to clear my knees when the oar blades are out of the water at about tube height, or just a bit lower. Of course that depends on how far out of the water your tubes sit and how high up you sit. In bigger waves, it sucks to have your knees in the way when you need to get the oar out of the water. In mellower water, the towers can be a bit lower, but knee knocking is the key for me and what forced me to higher towers (I sit on cooler on raft).
Living in Montana, boating in Idaho