I don't think there is a "one place fits all" for soldering on
posts. If the design is circular and rotation is not a problem, put
the post in the center. Offset the post toward the top of the
earring if you want it to always hang a certain way. Just don'tput
the post real close to the top edge - allow some coverage forthose
whose pierces have stretched over the years.
I agree with your assessment on non-oriented circular earrings with
only one caveat: As the diameter approaches about 1/2 inch, I find
weight may cause the earring to tip even using the larger clutches.
This is especially true if the design includes a stone.
In such instances, I prefer to place the post slightly off center,
preserving a sufficient margin for good coverage. This will nudge
more weight below the level of the post, allowing the earring to sit
better on the earlobe.