I use a Peak focusfinder and stop down to my preferred f-stop for exposure. I used to use a blue filter on my older LPL focusfinder, as described in Gene Nocon's book, and found it actually did make focussing easier for me.
If I have to use a small fstop, I'll focus a stop or so above (so I can see the grain better) and then stop down; but these days, my exposure/processing/printing technique is pretty standard, so it's rare that I need to use this method - usually it's saved for my earlier work. :)
Rough focus by eye with the lens wide open and filter in place, then fine tune it with a grain magnifier.
Often followed by forgetting to stop the stupid lens down before exposing... :o)
I've been in the habit of checking focus after stopping down but I'm probably just obsessing too much when I do that.
I spent a small amount of time yesterday with a blue filter, no filters and filters in the light path and the blue filter wins hands down, but probably not for the reasons people have mentioned.
Prints done using the blue filter on the focus scope were [to me] sharper than those focussed without the blue filter.
YMMV but for me at 60 my eyes are not what they were at 40 and for no other reason the blue filter provided a far superior delineation between critically sharp and acceptably sharp. When I checked the image on the baseboard there was no difference after focussing with the blue filter when I compared it to no filters or the normal dial in DeVere colour head filters for focussing. Focussing as I normally would with or without filters but not using the blue filter gave a sharp image but not as critically sharp as using the blue filter.
I tried this same experiment 20 years ago with absolutely no difference at all either way, so to see such a difference now is most likely the deterioration of my eyesight over time.
I will continue to use the blue Peak filter as a focussing aid for my aging eyes.
Filter on the focus scope, no correction filters in the light path then when focussed add the correction filters. And yes this is for Black and White multi contrast printing.
On another thread about grain focusers I mentioned that I would try getting some reading glasses from the drug store... I did and it makes a huge difference. Now I can see when my print is in focus... I'm happy now.;)