When I used to teach open water divers (in locations where we were doing drift dives and had no up or down line) I would get them to do the following: I did not mention a horizontal attitude at all.
My students generally learned very quickly to control ascents by doing what I said. I explained that I did NOT want them to be neutral, I wanted them to be negatively bouyant for the entire ascent.
The overall goal is to keep the students just SLIGHTLY negatively bouyant during most or all of the ascent. I tried to explain that the student was to use the most gentle kicking and small dumps in air. Keeping the students vertical allows them to easily see particles in the water, allows them to kick upward to maintain a very slow ascent and it is easier to sense an upward motion if you are vertical rather than horizontal.
When you want to go up, you simply start to swim up. Take a kick or two and stop and look at the particles in the water. If after you have exhaled, and you are sinking a little, then take another kick or two and exhaled for a moment and check. If you feel that you are starting to float up without kicking, then dump some air from the Bc and again look at the particles (and computer) and then start gently kicking. The key was not to allow yourself to float up, but to kick ever so gently. The second you screw up and sense that you are actively floating up, just dump some air and stop kicking. If the new diver can keep themselves always on the slightly negative side of the equation during the ascent it is much less likely to have an accident and runaway ascent. If you try to teach students to keep their face down and be perfectly neutral, this is much harder and because of the bouyancy of expanding air and wetsuit, makes the system an unstable equilibrium, it is much harder to do for the new diver.
Also, you have to consider what the horizontal diver needs to do if they dump too much air. Now they are (slowly) sinking and are negatively bouyant and the ONLY thing that will work now is to ADD AIR. If they do it too much, then you get the yo-yo ascent/descent cycle that is very disorienting and potentially dangerous. If the diver is in a vertical position and accidentally dumps too much air, they are in a position that a few gentle kicks will re-establish the ascent and they can potentially avoid adding ANY AIR to the BC on ascent. (This is an important distinnction in my opinion) If we can make it more likely that the student/new diver doesn't need to ADD AIR ON ASCENT then we are more likely to keep things in better control. (Not to mention it is easier to dump air from a BC in a verical position, for students/new divers that often have not mastered the rear dump.)
Of course, I would teach them to roll onto their back and flair out if things got out of control or they lost weights or something, but a horizontal ascent is not what i recommend for most recreational divers.
As a side note, I would explain to my students that they can continue to practice ascents and try to use less and less kicking (which means they do an ascent closer and closer to the neutral point).
Also recommending a horizontal ascent, requires that the diver do a bouyant ascent (or obviously they wouldn't go up) and recommending that new divers make bouyant ascents doesn't sound like such a good idea to me.
For myself, if students were not watching I will do a bouyant ascent and can control the positive bouayncy to keep it so minimal that it requires no kicking just breath control and fin drag, but this is not something I ever taught.
Staying ever so slightly negative on ascent is safer than staying ever so slightly positive, therefore I feel that a vertical is best for most recreational divers.
