MTB BBs are often installed with one extra spacer on the drive side, and I’m wondering if it’s the framebuilder or the crank manufacturer who should account for this asymmetry?
Should the BB shell be positioned symmetrically around the centre line of the bike? Or offset 1.25mm to the left?
The BB shell is normally centred on the frame centreline.
Depending on how you align and your build process, you may offset the BB a small amount, but this is just to account for material that’s removed when facing the shell.
The spacer you describe is really just there to deal with the ability to install a BB mounted front mech or BB mounted chain guide or similar.
Human bodies aren’t symmetrical. You won’t notice a 1mm or 2 offset in either direction. I make the BB symmetrical to the center line and then use whatever spacers are necessary to make the chainline work best. I don’t care if the cranks are symmetrical to the centerline of the frame.