In a scientific point of view, the most efficient shift point depends on the power curve of your engine. consider the two graphs below.
Graph 1
Graph 2
In the first graph you will see that the power and torque curves keep increasing with revs. This is typical of a low capacity high output naturally aspirated engine. But on the second graph the power and torque starts dropping after around 7000rpm, a typical turbo charged low engine.
So on car similar to what produced graph 1, it is better to shift at the highest safest rpm. But on the car with the second graph, there is no use of holding the revs after the point power starts to drop. Ideally you should shift at point the curve starts to fall.
My explanation is from a theoretical point of view. I'm sure there are more experienced members who will contribute with the practical aspect of it.