SpamAssassin DKIM with Virtual Hosting

[Jerry Malcolm]

I have a question about how SA's DKIM rules apply to virtual hosting.  
If "myhosting.com" hosts and sends mail for "JoesFlowers.com", does SA 
check the signature for "myHosting.com", for "JoesFlowers.com", or 
both?  Or is there some criteria to determine which domain name should 
have the DKIM signature?  Is there a penalty score if one or the other 
is missing?