I guess it's somewhat useful to memorize and understand that. What would be more useful though, for testing purposes, is to know the mechanisms that result in each dislocation. Posterior dislocations are relatively rare, and require a lot of force in the proper direction to occur. Generally, extreme generalized muscle contractions of the shoulder girdle can provide the force vectors and stabilization to result in a posterior dislocation, so if a question scenario involves tetany 2ndary to seizure or electrocution, you're likley dealing with a posterior dislocation (and a concominant fracture of the scapula on X-ray). If you're dealing with trauma, anterior dislocations generally occur because they meet less resistance.
Think of it this way, you have a ball on steep hill and u put a heavy weight on top so it only pushes vertically down. The ball will not be pushed into the ground, it will roll diagonally down the hill. Take the same ball and put a heavy weight on top, and also apply force towards the slope, it's gonna sink into the ground, not roll diagonally down the hill.
