do you use a filter on your 50L 1.2?

I have always been one to think that it is best to not use a filter on a lens just for the sake of protection since any added glass could degrade the image quality (with the exception of when using ND or effect filters).

However, with my newly acquired 50L I see that the front barrel actually moves in and out which could easily pull dust inside the lens. The manual actually says that a filter is needed to ensure dust- and water-resistant protection.

So, I'm wondering how many people who use this lens regularly actually use a filter on it? If so, which is the highest quality one, or the one that you use?

Thanks,
Ryan