I always use filters even though I only have one Leica lens. The lenses are in great condition, and I often shoot in dusty locations, so I'm sold on the added protection for the front element. Mostly they're B&W skylight or UV filters, although I got some clamp on filters for my Elmar 50/3.5.
FS: Pentax MX, Voigtlander Ultron 40/2.0 SLII in Pentax K-mount, Takumar 100/2.8 and 35/3.5 lenses: See my ads in Classifieds
M3, M2, R2A, IIIc, IVSB2, & T, and assorted LTM & M lenses
Minolta XD11, Pentax ME Super, and assorted MD Rokkor and Takumar lenses, Rolleicord III, Rolleicord Vb, Rolleiflex Automat MX-EVS