Necessary, absolutely not. My TF DC-3 had two wankels, both turning in conventional direction with no issues. Also, I have seen Ziroli (1/8 scale) DC-3's and one 13' w/s B-17 (four engines) with props going in same direction.

Having said that, in larger birds with larger, higher torque gas engines, counter-rotating props would make it easier on the rudder during taking off or acceleration/climb.

You may also hear about the OEI (one engine inoperative) advantage of engine torque counteracting asymmetric thrust if you have props turning inward (opposite of what Rich describes above). For example, if right engine quits, and left engine is turning inward (conventional rotation), this left engine's torque produces left yaw (as with all normal single engine models) that counteracts the right yaw developed by the engine thrust being on the left side of the plane.

Again, when you use the word necessary, I have to say it's not based on experience.

- George