I have a few flush cut saws like that (and other designs) and depending on what the veneer wood is (and it's moisture content) they can tear the $#!+ out of the corners. Straight razors can also follow gain and pull out at the corner. Ask me how I know...
A decent carbide bit in a regular router works nicely. Laminate trimmer is just easier to handle.
Sanding it in, as you originally suggested, is good if you have cut the veneer really close. It is very easy to take the corner out of square when sanding, but it can certainly be done. For a one-off (to avoid buying a router or trimmer) it's probably the way to go. But you really have to go slowly!
I may be revisiting this skill set soon to recover the SX-880 box. I'll need to reconstruct the corners a bit first, but it'll be fun to try real wood (unless I go with vinyl again).