Mitred edges - 12mm travertine

[Lodger]

How about grinding a pencil edge onto the reveal tiles, and hone. then put a miter on the 4 internal corners. So tile the wall first, with tiles flush or with 2-3mm overhanging into to the reveals, then fit the pencil edge tiles onto the reveal flush or with 1-2 space for grout line . All you have to miter is the 4 internal corners of the reveal. Not the whole face or it. That imho, would give a clean edge with a look you would achieve with a round trim. But without a trim!

 

[AD Ceramics]

Trims and sending the tile edge past!!!!!!! your a rough lot, thank god for leatherface coming up with the best way forward, mitring is OK if you cut the tile square on the wet saw and then do the mitre, that will give the best finish to the mitre.
Personally I polish all edges to external corners, provides a much better finish.