Yeah, very common and very standard in bike shops BITD. I did it all the time. To do it professionally, normally a shop would do something like the following:

A Park frame straightening tool (AKA Zombie tool) would be used to pull out the rear triangle one side at a time. I guess it could be done with hands, but if you pull the two sides apart at the same time, often one side will bend out more than the other. That's why the next tool is used, a Park frame alignment tool. You could use string, but a bike shop would not have. If you do, use fishing line or thread. That twine on Sheldon's page is not going to be very accurate. When the sides are both moved out the desired amount, 2mm each in your case, and checked with a ruler and frame alignment tool, then the dropouts are realigned, using ... dropout alignment tools. Lastly the derailleur hanger is realigned with a hanger tool. Everything is double checked, and the bike is ready for new wheels.

I realize you don't plan on doing this yourself, but it does give you a list of tools to ask about.