Ive been getting asked this question lately at the office so here is my opinion- In an ideal world, all upper cabinets would line up perfectly with the lower cabinets, naturally ending at the same point:
However not all cabinetry is built that way so I like to keep splashes ending where the cabinets end not the counter so that the splash doesn't look like it's hanging off in midair....
I found an instance where the subway tile splash was extended out diagonally. My approach to this would be to end the splash where the cabinet ends and extend the chair rail to meet at the backsplash:
In terms of bar areas, butler's pantries, vanities, I like keep the back splash contained to just the main facing wall not the sides. LIKE THIS:
NOT LIKE THIS:
Again, I prefer things to look cleaner and more buttoned up so in this case I would've installed the splash along the main wall (not the sides) so that the tile doesn't look like it's just "floating" out into the next wall. So that's it. My take on this tiny but important detail. Thanks for reading!