There are all kinds of hypothetical optical issues one could discuss. The main point, however, is that most enlarging lenses are not ideal wide open. You might need full apterture for general composition and approximate focus, but silver grain or dye clouds might not focus crisply at full aperture. So it's
a good idea to refocus a stop down (not necessarily at working aperture, which might be too dim to
focus well - just depends). But never assume a focus device is itself correctly aligned. You need to test.
And nothing will make senses if your enlarger is not perfectly aligned on every plane, or if your negs
are not truly flat in a glass carrier. I don't want to open a can of worms in the latter respect, but at a
minimum, you should have a glass carrier on hand with a focus target for the sake of general calibration. Some ten buck Home Cheapo level isn't the ticket.