If you do shave them, you will not completely lose your passage for air, because right behind the fenders, there is an opening into that cavity that will allow air to flow. But at driving speeds, the vents to provide "fresher" air, so to speak.
I want to shave mine as well but I got factory AC in my truck. I know there are other air entry points but Im thinking its gonna make the AC work harder. Dang AC dont want to BD it either.