There are two principles at play here: fit and aesthetics. For pleasing aestheitcs, handlebars must be rotated to adhere to the "rule" (which changes over time and local peer review). For many, this means the straight portion of the lower drop must be horizontal. Or pointed at the rear brake mounting bolt. Or some point in between. Or the slope into the forward rounding (above and behind the brake levers) must be level or some other chosen angle. (Modern bars often have so little straight at the lower drop ends that the concept of "horizontal" is approaching meaningless.)

Then there is fit. Some of us are passionate about the rotation of the handlebars being perfect for comfort; allowing for all day rides and hard efforts, even maximum efforts, without injury, pain or chronic issues. I"m in that category. The end result almost always ends up between the brake bolt and horizontal. I often mark that place with a piece of tape, but only so I can pull the bars off and replace exactly where they were or have a reference so I can do mid ride adjustments to rotation and not loose my starting point.

An aspect of the fit - some of us age and acquire injuries, chronic issues, etc. from time spent in sports, the military, industry, with power tools, riding bikes .... and have issues that mean handlebars that adhere to the rules physically don't do well for us or perhaps not work at all. If "perfect" bike setup is really important, I recommend staying away from such activities (including riding bicycles).