I think it is a matter of individual preference that should be based on personal experience of practicing at lease two past/mock exams to time. Either way, time must be strictly allocated to each section and, within section B, to each requirement according to the mark allocation.
One possible advantage of starting with section A is that it might be completed in less than 54 minutes – leaving more time for section B. However, there is a risk that candidates then rush section A in order to achieve this goal – and rushing makes for carelessness.