Juddian has got it spot on. I cant speak for other trainers but we have our vehicles set up to make use of retarders where this is helpful. Some vehicles simply make a lot of noise and achieve little whilst others will virtually eliminate the use of the footbrake if driven correctly.
The use, or otherwise, has no bearing on the driving test.
As Juddian said, a lot of this is best left until the new driver is in a position to experiment with different loads and vehicles following the test pass.
Personally, I dont spend more than a few seconds on the topic as there are so many variations a little knowledge could be less than useful.
Pete 