The "rule" about having the tips of the brake levers touch a line tangent to the bar's lower run isn't universal. For example, FSA specifically recommends having the brake lever tips 2-3 cm above that line for their bars.

As noted, what works for the rider is more important than following any specific rule but there are limitations. Campy specifically says their Ergo brifters have to be positioned where the bar's curvature closely matches the curve at the back of their hoods to prevent distorting the lever body when the clamp band is tightened.