Opinions differ.
At one end of the scale:
-Nonono. Puitting a 26.0 bar in a 25.4 stem will put a crimp in the bar which will cause premature failure at the worst possible moment. Prepare to kiss pavement and make your dentist rich.

Middle:
- Doable, but not recommended.

Middle, with detail:
- It's OK if it's a two-screw face plate, as it'll flex enough to avoid crimping. It's not OK on a 4-screw faceplate.

Other end:
-yeah, it's fine. It's a bike for crying out loud, not a nuclear reactor containment vessel.

I've used an adjustable reamer to turn a 25.4 stem into a 26.0 stem. Takes minutes.
The trick is to insert something between stem + face plate and then tighten the screws to hold the face plate in position during work.
A brake cylinder hone will take ages.
Sanding works too, but you need to take care to keep the profile round. Wrapping one layer of fairly fine grit abrasive paper round a 25.4 bar would probably work OK.
Or a coarser grit around a 25.0 piece of tubing. or...