Looking at the car from the front, you turn via the nut on the end of the crankshaft. The proper direction as seen from the front is clockwise. On the 6 cyl. M103 motor, this nut uses a 27MM socket. Not sure about other 126 cars.
I cannot speak for going the wrong way on MBs, but when this is done on Asian models with rubber timing belts, you'll sometimes cause the belt to jump one or more teeth, thereby throwing off the valve timing.