No. A car that normally understeers can still lock its rears first if there's a problem in the brake proportioning system. Oversteer and rear wheel lockup are different and un-related issues. If the rear wheels lock first, its not going to matter HOW good the tires on the back are, short of hot racing slicks with the consistency of chewing gum.
Better to get four matching winter tires. If you have two winter tires and two all season tires, one end will have much better grip in snow or ice, while the other end will have much better grip in non-snow / ice. That can make for some strange and surprising handling characteristics.
One of the theories I've heard is that with regular tires, deeper tread might as well go on the rear. The typical FWD car has most of the weight over the front wheels. With less pressure on the rear tires, the deeper tread helps the lighter rear end "bite" into snow or reduce hydroplaning. Don't know if it really helps with snow conditions though. Most FWD cars are supposed to have chains on the front. Most problems in snow are because drivers go too fast for the conditions.
Some go with narrower tires for the winter. Helps increase the pressure on the contact patch.
I'd go with all four.
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