Indeed, vignetting should not be a problem with a decent enlarging lens. However, it may be a problem with the light source, in which case stopping down will help.(posted 9560 days ago)Also, if the lens is flat field, and the negative is flat, and the negative, lens and paper are all parallel, there is no need for a depth of field. However, some (cheaper) lenses are not flat field, and stopping down will help. Another trick here, assuming the image is bowl-shaped, is to not focus on the centre of the image, nor the edge, but a point in between. This gives the DoF the greatest chance to get everything in focus.
And, at the risk of stating the obvious, I assume the previous reply referred to lenses with an aperture around f/2.8. Larger-format lenses tend to have smaller maximum apertures, around f/5.6, and correspondingly smaller "best" apertures.