Haggis,
When I trimmed my cladding for my tires I used a Dremmel tool with a cut off wheel attached to it. After the initial cut I used some sandpaper to smooth out and ease the edges for a "stock" look. If you didn't know it, you would never know that the cladding has been trimmed.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless