Question : Domain Keys. one key for entire domain or new one for each subdomain?

I am installing DomainKeys into postfix.  I need to add the following lines for example to my DNS zone file:

default._domainkey IN TXT "g=; k=rsa; t=y; p=MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAJQfGTmsFzILU6ep6aSFg+WrTkaOLmoRillFNbOpNOr5Gst5H8wG9Oh2SpUytaruP/7j/eWQ8Wyz6zX2gAtzwF0CAwEAAQ==" ; ----- DomainKey default for example.com
_domainkey IN TXT "t=y; o=~"

If I have 3 servers www1.example.com,  www2.example.com, and www3.example.com all sending mail that requires DomainKeys, do I need to add a different key under www1, www2, etc? Or can I simply add the one key to the main example.com DNS zone file?

Answer : Domain Keys. one key for entire domain or new one for each subdomain?

You do not need to add it for each host.

This setup is for the entire domain so one key per domain is all that is required.