For sending a new message there is no default id. You can howevere use the new mail form, where the id drop down box in right on the mail window below the Cc field. Set that is Tools -> options -> Messages and replies, the "identity selector" checkbox.
For reply and forward you can set a default identity for each folder. So when you filter specific incoming mail in a folder and reply from out of that folder the identity set as default in the folder will be used. Rightmouse when you are on a folder in the folderslisting to set it.