I'd suggest to avoid flat surface vertices as much as possible, the only "rule" that would allow to let a little vertex in the middle of a flat surface would be "avoid tiny-long triangles", else, there's no need to put vertices in the middle of a flat surface
Yes. It's better to run edges across the surface than to create a pole in the middle. From a rendering perspective one of the least efficient things you can do is have more than one triangle in a screen pixel(technically 2*2 pixels if we're being picky). Poles and thin triangles make that happen so they should be avoided.…