There are times when I know it is better to stay off the road... ice storms, blizzards, -50 degrees come to mind, but others will have their own criteria for safe cycling.
My advice is: be ultra vigilant, get studded tires, have multiple lights and multiple reflectors, and wear clothing with reflective patches. And use a mirror. Be ready to quickly dive off the bike if necessary. And if you are ever in doubt, immediately get the hell off the road. Snow plows may not see you but you can sure=as=hell see them.
Sadly, as a bicyclist you are the most vulnerable road user, but also the most maneuverable and adaptable to different conditions. No matter how safe a Swedish made SUV with FInnish tires, a bicycle is still the only winter vehicle that can be pushed beside you on the sidewalk or in the ditch when the road is unsafe.