I think this really depends on the lens and camera body. But, inIn my experience, with Nikon I have noticed that some of my lenses won't allow me to manually focus unless I switch the lens's focus to manual (as insome Nikon lenses, the focus ring won't move or it won't actually engage the focus). So, personally, unless the switch on the lens is set to manual. So I personally find it easier to just leave the camera body set to AF and toggle the focus on my lenses. I've I've been doing this for years with no ill effects on either the lens'slenses' or camera'scameras' auto focus-focus systems.