The companion web page Print to a specific printer demonstrates how to select individual printers without affecting the default. This page provides simple tools to store the preferred printer in the document's properties and to print to the named printer without affecting the default printer.
The functions incorporate modified versions of vba routines borrowed from the MVPS.ORG web site - specifically http://word.mvps.org/FAQs/MacrosVBA/AvailablePrinters.htm and http://word.mvps.org/faqs/macrosvba/MixedDocProps.htm to list the installed printers and to write to custom document properties - and are offered in the form of an add-in which you can download.
The add-in is packaged in a standard ZIP format containing a pair of document templates, There are two versions - one for Word 2003 (which will work with Word 2007 and 2010) and another for Word 2010 which should work with both 32 and 64 bit versions.
Extract only the appropriate version to the Word Startup folder. This folder is defined in Word at Tools > Options > File Locations > Startup, in Word 2007 at Word Options > Advanced > File Locations > Startup.and in Word 2010 at File > Options > Advanced > File Locations > Startup
On starting Word the add-in presents a toolbar. In Word 2007/2010 it adds two entries to the Add-Ins tab (if no other add-ins were installed, the Add-Ins tab would not previously have been displayed).
The illustrations are from the Word 2010 version. The alternative version is more basic (but functionally similar) but uses vba message boxes instead of user forms.
The first option 'Associate Document with a Printer' reads the list of available printers from Windows and presents them in a list box (see illustration below). The currently active printer is offered as the default selection. If a printer is already associated with the document, the associated printer is offered as the default selection. If the associated printer is not available, no printer is offered as the selection.
The function then writes the printer name to a custom document property 'Printer'. Custom document properties (and a whole lot of other stuff) are best viewed with the Bookmarks and Variables etc add-in.
If Cancel is selected, a message indicates whether the document will be printed to the active printer or the associated printer dependent on whether there is an associated printer and that it is available for use.
While it would be possible to intercept the print command, I prefer to offer a separate command to print to the printer indicated in the custom property field. The second command on the toolbar looks for the custom property entry and, if it contains a printer name, will check whether that printer is available.
If there is no custom printer property of the named printer is not available, it offers the opportunity to print to the active printer or to cancel. The macro finally reports your choice of action.
Word does not store information about the specific printer intended to print the document, and as the current printer to a large extent determines the formatting of the document, it would be useful to be able to store this information and set the active printer to the printer stored in the document.