If that is the case then I see even less reason to insert clean notes into received emails.
Personally I don't trust any received email received signed or otherwise, whilst the inbound emails that are scanned by avast if there was a problem then all hell breaks loose and you know there is a problem. I certainly take no notice of emails signed by other AVs on sending.
So if you are only intending to scan emails with attachments then personally I wouldn't bother with the clean note insertion.