Unless one is using stem direction to indicate voices, the most important "rule" is to use judgment in determining whether other rules "work" in a particular case, and overriding them when they don't. Most rules take a fairly narrow view of each individual problem, but in many cases other considerations may be more important.
For example, if there are many groups of beamed notes in succession, readability may be improved if the beams can be made to follow a reasonable contour, keeping beam directions consistent when practical. If the above message appeared between a bunch of beamed groups of low notes, and a bunch of beamed groups of high notes, it might make sense to have the preceding groups be beamed above the notes, the following groups be beamed below the notes, and the one group shown above slope down and to the right "between" the notes.
What's important is not to look at each beamed group independently, but to figure out an overall layout that visually makes sense. Oftentimes, there will be some beamed groups that could beamed reasonably well in several different ways if viewed only in isolation, but some of those ways would fit better than others in the context of surrounding groups.
Note also that an editor who is working as a member of a publisher's staff may need to adapt a more "mechanical" approach than someone who is working independently. If a publisher might end up having different parts of a piece laid out by different people, having those parts appear consistent may be more important than having them be maximally readable. If one is working independently, however, then trying to figure out what layout would be more readable would be more valuable.