I've seen a variety of things such as ALL amber pistils, first sign of amber trichs, specific dates, etc.... I think it all comes down to preference.. Muddy and I believe it was Andy B both say that for autos, trichs may not turn amber in some strains, so they may not be the best indicator, but the leaves dying is.. For photos, there's a bit of a line you walk:
clear trichs = highest THC, lowest (if any CBD)
Milky trichs = full maturity of trichs, a balance of high THC/CBD
Amber trichs = THC degradation, CBN becomes present, CBD (I believe), is highest at this point.. That's where the couchlock can come from..
So your call..