I believe you can get just clear glass filters to protect the lens by Hoya, which are probably not that expensive. Personally, and I am sure some one will correct me, I would look at what you use the lens for, and that make decision based on that.
As an example a lens I use for landscape is covered by a Skylight, and ND filter, where as my macro has a UV. This works for me, as I don't I have an in depth knowledge of filters, as I have probably just shown!!!
It is something I am interested in getting into though, in the next step towards world domination, in the photography world at least.