It's not absolutely necessary to change the rear end, but, you would then have to carry two spares, or use the dreaded wheel adaptors for the rear. You can find stock steel wheels in a 14 or 15" that will work with disc up front, and still look fairly stock for the rear if you're just using hub caps and not full wheel covers. Just get lower profile tires for the front and/or taller 13's in the rear, to keep the rake angle of the car within reason. The main reason to go with 15" wheels, other than the bolt pattern change, is cheaper, more selection, and easier to find tires.