Some enlarging lenses can be used wide open IF they are exceptional quality and significantly longer than "normal" focal length per format size, so that you are using only the center of the lens, and IF your
carrier and paper plane are precisely aligned. I sometimes do this with a 150 Apo Rodagon and 67x7 or 6x9
film. But with more typical lenses and applications its a pretty bad idea. The manufacturers give general
recommendations, and for most enlarging lenses its one or more frequently two stops down from max.