As you may know, you can auto-quit the printer driver when printing is over, so that the icon won't remain in your Dock.
However, in my case, this never worked. At least until now, when (for independent reasons) I changed the Default Printer pop-up in the Print & Fax panel of System Preferences to Last Printer Used. Bingo! My printer icon now quits the Dock when printing is finished!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071213011320331