Tires: two snows, or need all 4?, winter tires, snow tires

Question
I have a 2008 Toyota Sienna LE FWD van. I'd like to buy/install 2 snow tires up front. A tire dealer told me I need to have snows all the way around; all four instead of two. The dealer said if I only had the two up front, on the drive wheels, the rear would have a tenancy to fishtail around and not track properly. Do I need to replace all 4 tires with snows, or can I safely install just two up front?

Answer
Richard,

If you are going to put winter tires on a vehicle, not only does the vehicle have to get going, but it also has to stop - and winter tires are needed for both conditions.  

Now that would seem to apply to RWD, AWD and 4X4's, but what about FWD?  The rear tires also should be the same.  The theory is that if you lose control of one end of the car, when the vehicle slows down, the traction will return.  If you lose control at the front, when the traction returns you'll be pointed the right direction to avoid what is in front of the vehicle.  But if you lose control of the rear, when the traction returns, you'll be pointed in the wrong direction to avoid anything the vehicle is headed towards (backing into!).  So the general rule is that you want the best tires on the rear regardless of the type of drive.