Thirty years experience has taught me to use UV/protective filters on every lens at all times. Over time filters have taken knocks that have prevented damage to front elements and to the filter thread which is surprisingly easily damaged. I replace the UV/protective filters every few years as they degrade with fine scratching from repeated cleaning with occasionally less than perfect lens-wipes like handkerchiefs, shirts, ties, a best-man's coat tails, tea-towels, tissues or whatever falls to hand when you need an instant fix. I don't worry too much when the shot of the day beckons...the filters are relatively cheap and user replaceable. Even the front element of a 300 f/2.8 is deliberately not an optical lens, it's relatively inexpensive to have replaced by Canon. So use your shirt if you have to.
Tests I have done using new, high quality filters show no discernible IQ degradation when compared to filterless. Optics scientists with too much time on their hands may have tools to prove a difference, but in the real-world of day to day image making, experience has proved to me that it's always best to wear your protection.