James,
The NJG did use tracers albeit at about 10-15%. However, it was a pilot's choice as to the amount if any. Given that large amount of attacks were below and behind, seeing the tracer was not a major concern. As for the RAF, many gunners did not want tracers as they felt it would reveal their position. In most cases intercepts were made from 100-150 meters so tracers for aiming was rather unnecessary. The source of my explanations comes from reading several of the personal accounts and diaries in books. I am not certain about the RAF's nightfighters though not much said about this.
That is why most of my planes (NJG) have tracers set to 10-15%. If I use the AVHistory method of guns, then I can set each gun at a different rate and load different ammunition types (HE, AP, or Incendiary), but this needs to be designed into the model- not sure if you can hexedit this...hmmm. This adds to the fun!
Pauke Pauke!