As David said I align every print. I got the idea from Les because I was too stupid to think of it myself.
My setup is a Zone VI enlarger and I usually print 16x20 and 20x24. I usually print with the lens wide open because of the enlargment times needed.
I have a homemade table that allows the "shelf" that the easel sits on to go from about 2-3 different heights. When I built it I was pretty anal about getting everything level ( on each level) but after I got the laser alignment tool I found that I was always out of alignment.
So rather than fight the thing, I just align every print which takes about ten seconds. I then shim under the vacuum easel with those little wood shims that you can get at a hardware store.
If I was enlarging at a better f stop, this may not be necessary but until the LED enlarger head gets here that's the procedure.