1. Any brand UV filter should be fine as long as the size fits. Not sure about which verison of 18-200mm you have. The first generation Nikkor 18-200 filter size is NOT 72. You might want to chek on that.
2. Are you asking about min. focus distance? Every lens has one, not sure the spec. for 18-200. If you place your lens too close(less than its min focus distance) , then camera won't able to focus hence a blur photo. Also, if depends on the setting, the camera may not release shutter if not in focus.
Personally, I always have a lens on the camera. You never know next photo opp. arises:)
