SoC seems to be a common best practice recommendation ... or is "design principle" somehow different from that?

I simply meant that I didn't read it somewhere, nor can I cite some well documented philosophy; it is just the way I've evolved to do things.

The problem I have with most of the guidelines, rules and philosophies, typified by those in your links, is that they become ends unto themselves -- Is your application XYZ complient! -- rather than being used to guide, they become hard and fast rules that must be followed. Even when that it to the detriment of the application requirements. And that's a crock.