Postings with no charset, can one set a default?

I have recently changed my tin charset from iso-8859-1 to utf-8, I'm
not sure if this has changed what I'm seeing or whether it's just that
I have been looking harder, but ......

If a posting has no charset indicated in the headers I'm seeing ? for
accented characters and such, whereas if the charset is set correctly
I see the correct characters and symbols.  Is there a way to set a
default charset that tin will use if none is set?  I suspect that
setting a default of iso-8859-1 would probably make most of the ? I
see disappear as in the main I'm looking only at french and, more
rarely, other European languages.

-- 
Chris Green