The traditional style was horizontal, or a few degrees short of horizontal.
Now, with most of the pros riding frames that are too small, they need some way to make the bar easier to reach without resorting to the faux pas of flipping the stem. So they tilt the bar so everything, including the brake levers, is pointing upward a few degrees, just like casual riders who had ten-speeds but didn't like riding them in the 70s.
My bike is on the small side but I'm a bit of a traditionalist, so I tilt it to form a pretty much horizontal line from the brake levers through the top of the stem. This puts the drops at a few degrees short of horizontal, giving me a range of hand positions that I can use without resorting to cocking my wrists.