I find it somewhat interesting that kings and others slept in this way. It seems to me that sleeping in the usual, horizontal, fashion would allow one to quickly roll to the side if attacked. To be sure, it is easier to pin someone who is flat on a bed, but by the time you could do that, you could have stabbed them to death in any event, making the issue somewhat academic.
Additionally, a person leaning at an angle would require a less time-consuming movement to stab or hit with any sort of weapon, assuming that the assailant was standing. That would mean that it would be more dificult to dodge the blow, and a guard would have a second less to intervene.
As for the Minbari, just to clarify, I seem to recall that they saw sleeping horizontally as a bad idea not for purely psychological reasons, but because they had a superstition that it invited death. Thus, it would be akin to a human sleeping in a dangerous setting, rather than a merely morbid one. When Delenn, for spoiler-laden reasons, had occasion to sleep in a human bed, she seemed far from disgusted!