Have to agree Peter that f22 is usually the best aperture with most 5x4 lenses when you want optimum sharpness, and that's fine for tripod based work, which was all my LF work until recently.

The problems arise when you want good sharpness hand-held where you have to trade apertures against shutter speeds, I'm often shooting in locations now where tripods aren't permitted, so it becomes more important to know & understand how your lenses perform at wider apertures.