Question: In the Fiber Coupling Efficiency Calculation under Analysis...Calculation...Fiber Coupling Efficiency, when should one use the Huygens Integral and when should one not?

Use the Huygens Integral for the most accurate result. Its only disadvantage is speed, compared to the non-Huygens method. The non-Huygens method is somewhat faster. Therefore, test the results with the Huygens calculation on and off: if the coupling efficiencies are 'sufficiently' close (which is a judgment only you, the user, can make) then the non-Huygens result can be used with confidence.

You may also want to use the non-Huygens calculation for initial setup and optimization, and switch to the more accurate Huygens method for final optimization and evaluation.