I do this if I have the time. The mids, you will harvest more and better quality. Some growers suggest that the mids them become the better smoke. Unfortunately I have not been bothered to do it myself for awhile.
From what I understand THC is always being created and decaying . As
@Beachwalker said, look at the buds and not at the leaves and ill add No point looking at the trichs until the flowers are mature, doing so can make harvest time confusing. Look for all Brown (or on some strains vast majority) brown Pistils that are starting to recede (they curl up and recede back to the bud). That's when you grab the loupe if you need to get peak ripeness.
Ill have a quick stoned ramble.. I think that a short flowering time on a description increases sales as the vast majority of indoor growers want a short flower time. So it "can" be used to make extra sales = profit. It can confuse growers as most of our setups wont reliably see the days suggested (we see this everyday on threads) and then we harvest a tad early than what, in my opinion, we should be.