UV filter for L lens?

Currently I have 35L, 85/1.8 and 135L for my 5DII, and have B+W MRC filter on each lens. But I read some article said any UV filter will affect the photo quality, so I start to worry...

I knew this topic has been discussed many times, but I am still argue with myself and not very sure if I really need the UV filter?
Thanks for your advice on my concern.

Eric