Using a slightly wider enlarging lens isn't only about a loss of edge sharpness but a light fall off factor. When you print with a longer lens you are moving the enlarger lens further from the neg therefore making the distances between the center of the neg to the lens, and the edge of the neg to the lens more similar. You are reducing a sort of focus foreshortening. This can also make it easier to get good edge to edge focus on the neg. You are also projecting from a smaller area of the enlarger's light source, that is you are not using the edges of the condensers or diffuser but are using a smaller centered section. It's just more even.
The fall off factor is not as bad when you shoot B&W, where the fall off of the taking lens is somewhat equalized by the fall off of the enlarger lens, they work in opposite and balance eachother. The fall off of the enlarger lens means less light passes through to the printing paper on the edges, and as the edges of the neg are usually thinner than the center due to it's taking len's fall off, a balance is struck. However if you print color from a chrome, the fall off of the taking lens is added to the fall off of the enlarging lens giving you very noticeable fall off on the print.
Thanks.
