A common question that I am asked is if the keyway on the drive and the keyway on the driver shafts should be oriented 180° apart.
My response is twofold. If the unit is designed and manufactured as a single system, the orientation of the keyways should not matter as the key length will correspond to the bore length of the coupling (and the keyway cut into the shaft should also be only just long enough to properly fit the coupling).
However, some applications have a longer key & keyway than what is needed for the coupling. The longer key may cause an imbalance in the system if both keyways in the shafts are aligned. It is with this potential imbalance in mind that the “rule of thumb” to orient the keyways 180° apart was created.
Big picture, if you want to minimize imbalance in a system when installing a coupling the 180° keyway "rule of thumb" is certainly a worthwhile. While it may not always be required, it is a good "best practice" to follow.