Borders tend to be placed directly under a window cill where appropriate.
Here they a
Continuous under, rather than broken by, the window.
At a visible level above the sink taps.
A decorative framing for the window cill.

BUT

The bottom line is:

The best way for you is the way YOU like them!

I refurb'd my bathroom last year with a border of small marble pieces on
plastic backing (if you know what I mean). Not only did I run these
horizontally around the room, but also included 2 vertical full height
"borders?".
This gives the room a slightly modern feel without resorting to nasty chrome
tiles etc

Phil