The artful use of panning can turn a really good mix into a GREAT mix. But don't get carried away. Here's a great synopsis.
http://www.loopmasters.com/articles/2870-8-Tips-For-Using-Panning-Effectively-In-Your-Mix
http://www.loopmasters.com/articles/2870-8-Tips-For-Using-Panning-Effectively-In-Your-Mix