Didn't spot one nuance in your question. Only indoors in poor light do I use a lens with no filter fitted. Generally I use the filter that's already fitted to the lens, though occasionally I have to change it. I do always keep a lens cap on, to protect the lens both from impact & inadvertent touching, but also from the air which here is permanently greasy and salty.

Canon "L" lenses are hermetically sealed when a filter is fitted and they're either attached to a camera or have the back lens cap on. A pretty good reason for me to follow those basic instructions, especially when you see what can happen to unprotected glass down here.