In reply to Anonymous:
As stated above, it is very rarely shut. The thing most likely to shut it is the behaviour of drivers.
There are times however when visibility reduces to zero and there is no chance of it staying open. If you ever have to pull over and climb into your sleeping bag, make sure you are completely off the road (unless you feel comfortable with a 26 tonne snowplough up your ar5e).
Normally, even during moderate to heavy snowfall, so long as enough experienced Highland truckers keep going and the cops dont lose their bottle, it'll be fine. Loads of local truck traffic on Friday afternoons, so this may be a good time.
Once it shuts, the struggle is getting it open again. The bit in Perthshire is dual carriageway and twice as much work.