Well it is up to you if you scan inbound content, but personaly I would wan't attachments scanned.
Or you could exclude NewsBin from the [MailScanner] section of avast4.ini
[MailScanner]
IgnoreProcess=NewsBin.exe (or whatever the executable is called)
However, as I have said both these options could leave you more vulnerable.
I don't know how newsbin works, but what you may have is double scanning as usenet is imported using newsbin, avast may scan the content then and possibly again as it comes from newsbin to your newsreader (if newsbin is just an interface not a newsreader).
If it is double scanning, ignoring newsbin.exe? and leaving the scan inbound NNTP would still leave you protected. Unfortunately I don't know enough about newsbin to really say for sure.
A forum search for newsbin returns many hits, these are just one:
http://forum.avast.com/index.php?topic=14655.0
