Interesting discussion. I don't think I've "ever" locked the knee.

On both my mills, I trammed in the head by using a dial indicator
several inches out from the quill and adjusted till perfect in both X
and Y. I always watch when using a large face mill to verify that the
head in still in perfect tram. There's nothing that shows out of tram
quicker than a skim cut with a large face mill.

I can see where locking the knee might change this. If you're getting
good results with out locking the knee I think it would be best to
never do so.

However, I bet many machines out there have more wear at the center of
travel and using a face mill with the knee all the way down would
produce different results than at the center of travel. In this case,
always locking the knee would improve things.

My two cents

Karl