I'll take the temperature into consideration when deciding to use the studs or not. If its above freezing and they're not calling for much I am more likely to leave the road tires on. If its below freezing and/or they're calling for a considerable amount then I use the studs. But anytime they call for anything over a inch and its around 32f/0c then I'll put the studs on. I learned a long time ago not to take any chances. There have been lots of times when I could've probably gotten away with using the road tires. But I don't want to risk it. Sure, it might take an extra 5 minutes or so. But I'd rather be safe than sorry. I use Nokian Gazza Extreme 296's. There is nothing that they can't handle. They take on solid ice like its nothing. I've ridden in empty parking lots that had glare ice on them. And did a bunch of big 'S' turns and didn't slip once. They are starting their ninth winter. I can comfortably ride in 8" of snow. Above that I just improvise. The key to riding in deep snow is to keep spinning and moving forward. It may take longer but at least I'm moving. Practicing riding a straight line comes in handy for riding down a tire track when the need arises.
I have them mounted to a spare wheelset that matches my road rims. I use Rhyno Lites. It takes me less than 10 minutes to switch wheels. Lots of times I've made that last minute decision before heading out to switch wheels. Once you get past the cold, winter riding is like nothing else. I look forward to the challenges that Mother Nature throws at me. The weather around here isn't too bad. Nothing that I can't handle. I'm 59 and I know my riding days are winding down so I do it while I can. Besides its good for you.

If in doubt, put em' on.