Cold setting a frame is not uncommon. Sheldon Brown discusses it here:

http://sheldonbrown.com/frame-spacing.html

I've cold set several of my frames, the most recent a Surly Long Haul Trucker from 135mm to 145mm for a tandem rear wheel. I've got about 3000 miles -- carrying about 280lbs -- on this frame now after cold setting it.

A 10mm spread is less than 1/2 inch, and only 5mm (1/5 of an inch) per side. That is not much of a spread. If your frame is metal, most likely this spread won't be difficult or problematic. I wouldn't hesitate to give it a go.