When I built a Gentle Lady glider, the building instructions called for 1/4 or 1/2 , I don't remember exactly, inches of washout at the wing tips. In addition to making sure that the center of the wing stalls first, it also reduces induced drag at cruising speed. Because the wingtips are not generating much lift at cruising speeds, they also are not generating high energy tip vortices. Too much washout can have the wing center and wing tip lifting in opposite directions at high speeds which increases drag.