When do you all normally put them on?
Depends. I usually get my winter tires put on as part of annual maintenance. They are always on by Halloween because by then we've had a few frosts and sometimes even snow that sticks to the roads -- and because I run summer tires, not all-seasons, so I've got
no grip on ice.. Some folks have the ability to swap tires right in their own garages, so they may wait until late November or so to swap all-seasons for winter tires.
Do you rotate snow tires more often since they are softer?
You could, I guess. But, really, snow tires aren't any softer than pure performance summer tires. Would you rotate those more often? Besides, weather does not change the car's dynamics. On front-drive cars, the front tires still will get the bulk of the wear from starting, steering, and stopping the car.
Then there's the part about making sure the better tires are always on the back. Putting the tires with the most traction up front is one of the fastest ways I know to make the back of the car the front of the car. So at some point you really won't be able to rotate them safely.