When do you use filler wire?

I've been getting into MIL spec wiring and have done a couple of small harnesses now. I'll admit I've not done the wiring course yet.

I wonder when you guys will choose to use filler wire? EG, on a recent small coil conversion harness I did to interface with an OEM engine harness, the second concentric wound layer had 5 wires - I left it at this, but I wondered if I should have added filler wire to take it up to the full 12?

This is finished winding before fitting the DR-25. Do you guys have any rules of thumb for adding filler wire?

Is it the DR-25 TW that has the yellow print on it? The stuff I've been using has white print.

Came out looking OK I think. Damn that epoxy is messy if you are not careful!