I run mix-n-match tires all the time with no ill effect BUT... you have to use tires of very similar tread and tire compound to get a good result. This thread is a worst case scenario:
http://986forum.com/forums/general-discussions/31969-what-tire-pressures-do-you-run-street.html
He is running max performance summer tires on the rear, and all season (600 treadwear) touring tires on the front. This is a pretty big mismatch and the car understeers terribly. My brother ran a track day with A/S tires on the rear and Max Summer tires on the front. His car was so tail happy he put it in the dirt 6 times.
The bottom line: If you understand the difference between tread and tire compounds and are comfortable choosing 2 different tires of similar design, mix-n-match will work for you. If you are fully dependent on a tire shop technician for tire choices, you probably better get all 4 identical tires.