При создании DomainKeys имеет значение, если я использую o = ~ или o =-?

The o= tag at the _domainkey root is mentioned in this DKIM specification draft as an optional mechanism to indicate your outbound signing policy.

o=~ means "some, but not all mails from this domain are signed"
o=- means "all mails from this domain are signed (though not necessarily by me)"

Much akin to (though not to be confused with) the Fail and SoftFail qualifiers used in SPF.

Remember that this is your signing policy, you are the one to decide which policy indicator best meets your needs