The late Canadian photographer Gene Nocon wrote a book about printing and focussing the lens on the baseboard. He explained that because B&W images are only really interested in the blue end of the spectrum the focussing should be made with a grain magnifier which has a blue filter. He has images in his book demonstrating the difference between a non-blue filter focusing image and one where a blue filter is used. It does apper quite dramatic. The focussing magnifyer is one that used to sold in UK under the Peak name but was available under different names.
I happen to have one of these grain magnifiers but sadly no blue filter, so I use the colour gead on the LPL enlarger and dial in 50Y and 50C which gives a good blue light source. And yes I can support his theory that a blue filter does improve the focus point when using B&W. Obviously with colour neg there is no use for the blue filtration.
I focus wide open using my Nikkor 2.8/50 and then stop down to f11 for normal negatives and F8 for those a little denser. Corner to corner sharpness guaranteed.
Ctein's book "Post-Exposure" says that in his tests w & w/o blue filter, you're better off w/o it. I have a 1c and a Micromega 25x focuser--focus it once, and it stays in focus indefinitely.