However, I can't see the need to religiously stick to one method when other options clearly work.
Besides which, is it that important? Obtaining the result you're looking to achieve is the goal. How we choose to get there is our own journey.
The reason I am making a point of this, is because it's hard enough for a beginner to learn the details and workings of skinning as it is.
So any information added to threads like these should aim to be correct at least.
People will read your statement that using 5x5 image sizes is just fine, and may think it really doesn't matter much.
Well it does matter a lot if you want to get expected and controllable results.
Especially your suggestion for the top corners using 5x5 instead of 5x28 is very bad.
So in my opinion you are spreading incorrect and confusing information here.
That it doesn't seem to matter much for your own skins, or perhaps some other skins that have a single uniform colour, that's fine.
But others may have different objectives to get their borders right, and your suggestions can result in serious problems for them.
edit:
An example what can happen when you use incorrect image sizes such as 5x5:
(and this is a configuration panel, not the main panel border that you may or may not care about)