While Jim is correct, I find it much easier to align the enlarger to make everything parallel and then just do a spot check to make sure the corners are all in focus. Way to many options on what to adjust just looking at the baseboard for me.
Also, for your check of sharpness, make sure you use a glass carrier. Film flex will really throw you off otherwise.
Also, I use a laser and mirror instead of a level to check alignment. I place the mirror on the lens bezel. Much easier to use this way, and very easy to spot check before a printing session.