I am in "on and off" mode in this question. Sometimes I use a filter for protection, sometimes I don't.
To put it simply, for L glass, well. I paid for good IQ and to get away from the guesswork, if the situation is "non-threatening", I shoot without filter. However, as I do some sports shooting, let's say I am at the drinking station in marathon where a lot of splashing happens, yes I use a filter. Same thing while travelling, strolling streets of unkonown city and unknown situations, yes, I use a filter. However, if the situation is controlled, I am taking a landscape shot with a tripod for example, there's no way I would use a filter (unless, ofcourse, I am in the middle of storm or something like that). I do not know whether the filters would actually degrade IQ, except for my polarizer I should have top of line models, but at least in those controlled situations I do not have to guess.
Oh. About hoods, if you have one, that's one thing you should really use. I learned my lesson in one night shot session, where maybe 20% of wide angle shots had various odd reflections and such. Something that a hood could probaply have prevented. And obviously, it is great for protection. Don't leave home without it.
