I have two good levels, one 4' and one 2', both are identical and read the same when swapped end for end. I have a couple of torpedo levels that aren't in that league.
In light of the fact that most enlarging lenses are not perfectly centered, I am using the exposed/developed leader that had been abraded with 100 grit sandpaper as a test neg and that gives me very fine lines to check overall focus. Then, on the enlargers that have negative stage pitch and yaw, I simply adjust for best presentation. Of course, the enlarging lenses at 2.8 are a tad softer at the edges but are great down about 1-1/2 to 2 stops for final check.
I am contemplating renting a 6-8 inch machinists level to see if that is an accurate way to go. Still, the mirror setup is so quick when beginning a session and it gives me the confidence that I am starting on solid ground.
My worst enlarger, a salvaged 23CII that was stored in a shed for several years and severely banged around (a lot of sheet metal was out of square), has come right into alignment with little tweaking.
I am pretty happy with the setup but I don't know that spirit levels are any quicker/better but I want to do some experimentation with how far out can an enlarger before it is visible in a print. Ballpark guess: I would guess a lot of enlargers are out a bit but are still producing reasonbly sharp prints, especially at the 8x10 size. A 16x20 would scream at you if the enlarger was much out of alignment.
It could be that a bubble alignment/check is just as good when it comes to viewing the final print. My mirror setup is very sensitive and what little drift I see from session to session may be inconsequential.
Later,
Fred