I agree about the reliability of the NTS system. Norm told me that they did away with the test altogether on the -14s in the Cheyenne 400 because it was so reliable the test was deemed unnecessary.
And worse, the most likely failure is of the pressure switch used only for the test itself!
That said, on a Jetprop it is so easy to do (just flip a switch and watch the light) that it is a habit pattern for myself, and so I do it every time. On a 690, with the more awkward switches, I can understand not wanting to do it all the time, although even in a 690 it is one of my habit patterns to do it every time.
And, if you do the test, regardless of whether it is a once in a while or an every time, you should make sure the check valve closed afterwards by doing the check valve test.