which L-lenses need filters for weather sealing? Or, more precisely, do any of my lenses need filters for weather sealing: 14L II, 50 L, 24-105L, 100L?
The 14L II neither needs nor accepts filters on the front of the lens.
The 50L requires a filter for complete sealing.
For the 24-105L and 100L, it's ambiguous, but I do use filters on them (on all my lenses, actually).
The lenses that definitely require a filter to complete the sealing are the 16-35 II, 17-40, and 50L - those threee state that in the manual. The lenses that definitely do not require a filter are the supertele primes (200/2, 300/2.8, etc.) which cannot take a front filter. The other sealed lenses (with the mount gasket) have no requirement for a filter stated in the manual, but Chuck Westfall has recommended using a filter for the sealing on those lenses.
Hope that helps.