Hi--

>>It is clear to me. A $25.00 glass is not going infornt of my #400.00 glass
>just in-case!
Robert Kirkman

That's up to you, but making good optical flats is a lot easier than
making (accurately) any one of the many curved elements in your lens...
And a UV filter saved a favorite lens of mine when someone on the
street grabbed for (and scratched the UV filter on) a favorite lens.
The UV was cheap protection. I have tested to see if there is any degradation - there was none, even with a 400mm f3.5 (long and fast are
the two worst-case situations for filter quality) with none, front or
rear filter in place, or both filters in place... Not using one due
to the possibility of image degradation borders on religious belief,
not provable fact, IMO, so I put them in front of my $40, $400, and
$4000 glass...;-)
David Ruether