If you are referring to a UV filter, they do nothing with digital, except sometimes cause flare. Even the very best. If you want protection, then you have to weigh the balance according to circumstances.
I never use any sort of protection filter unless absolutely necessary, and you have one of the finest lenses ever made. It was intended to be used without a filter, and a cheap one makes no sense at all.
If you run a search there have been several threads on this quite recenly, some showing horrendous examples of flare. The problem is mainly caused by the surface of the sensor reflecting light back, which then bounces off the rear surface of the filter, and so on back and forth.