Just want to chime in with a bit of common sense to the OP...
To filter, or not to filter (as long as we're talking quality filters) depends, IMHO, on the quality of the air where you're going to shoot.
Those who say, "judge it by each situation"....yes, I can go along with that. But the sweeping statement "do not filter" really winds me up.
Because I live in southern Japan, right slap-bang next door to one of the world's most active volcanoes (typically erupting 3+ times per day, LOTS of highly abrasive ash, which floats all over the city). AND, we're not far from China, which equals some of the worst air pollution in the world right now - and it's blown across to here several times a year, depending upon prevailing winds.
So, PLEASE, don't accept blanket statements that filters are a "bad thing".
They won't protect your lens if you drop it - I expect.
But volcanic ash, sand, salt-air, particulate-air pollution...?
I have no issues with cleaning/throwing out a filter. But I DON'T want to clean a front element no matter how "tough" the finish is supposed to be.
￥8,000 versus ￥120,000~￥250,000 is, to me, to coin a popular-though-overly-used-Americanism, a "no brainer".
Regards (/mini-rant )
PS: Not a single one of my clients has EVER complained about poor image quality!