I keep a filter on all my lenses -- except recently some with deeply recessed front elements. I do it for two reasons:
1) I once lent a lens to a friend, he dropped it. It hit the ground front first. The impact shattered the filter and deformed the filter mount, making it difficult to remove. Once removed, it became clear nothing else was affected.
2) I can clean the filter and not worry about cleaning marks or contrast killing crud on the front element.
Through much of my time, I have shot slides, never side by side comparisons w and w/o filter, but, for my eyes, anyhoo, shots looked fine, and better than the effects of overexposing the slides -- just to calibrate the magnitude of the effect.