Almost always on my lenses, and I always have a spare around cuz I'm kinda klutzy and they're cheap protection - both optically and physically. I've replaced more than one.
(Side note to amfoto1 or anyone who has the EFs10-22 or EF17-40L: You can use the EW-83M instead of the frisbee that comes with it. Designed for the EF24-70ƒ4L, it won't 'click' in place, but it won't vignette at full wide, either.)
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