I use B+W ...either UV or clear filters on my lenses, for protection of the lens. I don't have filters on any of my three Limited lenses...21,40+70...for the simple reason that I've had difficulty fitting filters on these lenses. All my other lenses, I've fit filters on, with the obvious exception of the 10-17 FE...which won't take a filter.
So have 12 digital lenses...8 have filters...4 none. Eleven Pentax digital, one Sigma.
If you are going to use a filter, use a top quality like a B+W. What's the point of putting a cheap filter with a cheap piece of glass in front of your very expensive lens.
BTW, I've experimented taking pix with some of my lenses...with and without filter (B+W) and I find that I can't see any difference...even when the pix is blown up to 11 X 14. I don't enlarge more than that....we just have a regular size house and only so much wall space.