Interesting thread starting out with hoods and migrating to filters. If the premise is protection then both the hood and filter should be used to protect the front element. Would you buy a used lens with a scratched front element even though the seller claims it doesn't affect image quality? I sure wouldn't. There are countless accounts of how both a hood and filter have saved front elements from scratches and cracks from impact damage. If the premise is image quality then again you should use your lens hood. Manufactures design the lens hood for each lens to combat flare. If a lens didn't need it probably the maker wouldn't supply one, offering one as an option for an additional cost
Regarding filters and image quality: Why put a cheap or "less expensive" filter on an expensive optic..doesn't make sense to me. I only use Nikon filters. I figure that the manufacture is only going to use the best possible optical glass with multicoating so as not to degrade the image. I have read about the possibility of rub marks on front optics from over cleaning. If a filter becomes scratched or the coating rubbed off, its a cheap replacement. Bottom line for me is I always use the lens hood and filter.
Michael Rogers
www.imagesbymichaelrogers.com