I started off doing stops vertically.
Then improved my ability to hover by doing them horizontally.
Then as I improved my ability to control depth with breathing while staying motionless, I went back to mostly vertical for the safety stop. I find it easier to keep good awareness of the surface and people in all directions around and above me.
At first I had to look at my gauges a lot, but with experience you can get pretty good judging distances over the last 20'. So typically now, I look at my computer just as I approach 20'. Then I do a very slow glide from 20' to 10' over however many minutes I've chosen to do my safety stop, with just a glance at the computer now and then to check the elapsed time.
If you are neutral with little or no air in your BCD, and not a lot of neoprene, then with experience, holding a constant depth becomes automatic whether or not you are horizontal or vertical. The primary short term feedback is junk in the water, and feeling any motion in the water. The long term feedback is eyeball estimate of distance to the surface.