It's not much of a problem as long as it's both backs or both fronts. The real problems start when you replace only one tire or both on one side of the car.

The biggest issue with replacing the rears with a different tire is the level of grip in a corner. If you have really sticky tires in the front and really hard tires in the back you can have a "loose" condition, the rear end wants to slide out. If you have the opposite, sticky in the rear and hard in the front, the car will seem "tight", it'll want to go straight when you turn the wheel and give it the gas.

As long as the tires in front and the tires in the rear have a similar treadwear rating and level of grip you should be alright.