I hate Scott Bourne and everything he stands for with the exception of one thing...
Filters...
"why pay thousands of dollars for high quality optics only to put a $25 coaster in front of it?"
Not a direct quote but close enough...
I use filters on my most expensive glass, for protection.. and the sub $1000 stuff I just go commando.
I only use B+W's XS MRCs with the nano coating... they are the easiest to clean, the most scratch resistant, and optically superior to most other filters.
Lenses were designed to perform at their best with exactly the number of elements they are built with...
If you MUST add another element, don't skimp on the quality.