the tires with good tread should always be mounted on the rear of the car. If you buy two new tires, make sure they are mounted on the rear as they should be. If you buy 4 new tires, just be sure to rotate them regularly.
I just had to explain this to someone and they just gave me a look then told me that the good tires are supposed to go on the front on a FWD car. FWD, 4WD, AWD or RWD, it doesn't matter, the tires with the best tread belong on the rear for directional stability (especially around turns)