I attached all my lenses with a 1B, MC or UV filter. Hoya Kenko B+W do the work. This topic appear every now and then and always there are 2 camps of people. Some worry about degradation and some people like me worry more of our lenses =)
Until now i dont see picture degrade with filter unless you are using dirt cheap filter. Even a Hoya UV GUARD, It is not an issue. Today people use 30" monitor and zoom 100% to look into sharpness, but for me I use only a 24" and 90% of my print are only 4R/S8R.