The coatings on the front elements of 'L' lenses are very robust the only protection I think 'L' glass needs is a lens hood, any filters that aren't being used for a reason (ie polariser or ND) shouldn't be used in my humble opinion, unless of couse there being used as advised by canon to complete the water resistant nature of the lens (with a compatable body), ie 1 liter on the 70-200 when used on a 1series body.
Alan