I'd use something short.

Potentially-irrelevant example: I'm using a 107mm bottom bracket with a Sugino triple crank on an old Bianchi with a 70mm bottom bracket shell and 126mm rear spacing. It's set up as a double, using only the two biggest rings, and the chainline is still a couple mm outboard of being centered with the 6-speed freewheel.

My point? If the BB shell were 68mm, and I were using a double crank, I imagine the crankset would be perfectly centered with the FW, and that's using the shortest BB I can get from Shimano. You don't want to know how badly-aligned it was with the 123mm BB spindle that came with the bike.