Plants will continue to produce new trichomes up until a certain point, so you will never get them all at their peak... that's why so much debate over when to harvest.
When the heads turn a milky white color, the THC is fully formed. When they start turning amber, it's degrading into other substances.
The question is whether you want more clear and milky to keep it an up high, or more of a milky and amber to give you more of a "I don't want to move" stone.
Even if you let the plant go to 30% amber, you will still see a few clear heads every here and there simply because the plant continues to produce small amounts of trichomes as it ripens off. In the end it's all personal preference. The same strain harvested at two different times can give to different high profiles. It will still be similar, but certain effects will be stronger than others.