I keep UV filters permanently on all my lenses, both Nikon, Hassy, Rolleiflex, Pentax 6x7 and everything else I've forgotten I had at some stage; filters are ordered with the lenses. I can see no valid reason not to, and every reason for doing so. I have seldom been victim of flare, and I put it down to lens design rather than anything else when I have been, the only lens I can remember being guilty was the 4/150 Sonnar for the 'blad. And it was never as good as the 180mm Sekor it replaced when I gave up the Mamiya TLR system.
If there's a difference, with or without filter, then I accept that my snaps are not good enough to reveal it, and in any case, it would make no difference worth talking about to the end product. Just another version/level of pixie-peeper thought.
Far better, as others have pointed out, a new filter than a new lens or lens element!
Rob