I'm not so sure the answer is one or the other. It seems to me that there are too many angles of approach and therefore too many variations for the 90 degrees to apply universaly from the shoulder to the elbow to the wrist. I find that most of my shots are with a level cue and a 90 degree approach, from the elbow to the table bed, offers the highest level of precision. Others may have a different viewpoint.
Steeper angles may afford a more effective approach with the 90 degree angle from the shoulder to elbow to the wrist offering sufficient angle to allow for the stroke movement since the steeper the angle the smaller the range of motion between the upper arm and forearm....but not necessarily the most efficient stroke.
In its extreme, the masse shot is at the maximum level of elevation and there is no 90 degree correlation between either of the two options.
If we are given only two options and asked to pick one, we limit all possibilities that may apply to the other option. Perhaps both options apply in different scenarios.