I doubt it would be too warm at +5 Celsius, but it could be a bit too cold at -12, since the "lower limit" rating is -9. A lot would depend on whether you tend to sleep warm or cold in general, and what clothes, if any, you wear inside the bag. For winter use, you'd likely wear a base layer of some kind in the bag, and could, of course, add more clothing if necessary.

Personally, I mistrust the ISO ratings. The best way to judge the warmth of a sleeping bag is by the amount of loft (thickness). As far as I know, Western Mountaineering is the only manufacturer who states the amount of loft for their bags. A visit to their web site, westernmountaineering.com, may help you compare. If you can see the Mont Bell bag in person before you buy it, you could check it.