1. Do you leave your lens hood on all the time? I've had my 24-105 f/4L for about 3 years and have never taken the hood out of the box. I just thought it was for helping with sun flare and I just use my hand if needed. I never really thought of it as protection for the lens (I thought that was why I bought a UV filter). 2. I've read/heard that you are supposed to always turn your camera's power off before changing lenses. I've been good about it, but have always worried that someday I will forget. What happens if you leave the power on and remove a lens? Thanks so much!