This is not going to be the most exciting of all hints. But it is another aspect of using smart mailboxes I haven't found so far on macosxhints.
You can configure Mail to update its Dock badge (on the General tab of Mail's Preferences) for unread messages in either all inboxes or all mailboxes. But you can also select a Smart Mailbox to be responsible for updating the Dock badge.
For instance, create a Smart Mailbox called Daily that contains all your inboxes and other important mailboxes you need to track often. Then set the Dock unread count preference in Mail to this Smart Mailbox .
For most of my other mailboxes, it is sufficient to check them once in a while, but I do not want them to show in the Dock unread count. This makes the dock unread count badge much more useful for me.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090824044138480