Your post said "run" a spacer, so most folks probably assumed you wanted to ride it like that. From a practical viewpoint, the steerer would be thinner 10mm up from the crown race dia, & hence a bit weaker. You could get some sort of part turned, that would sit on the crown race & slim down to the req' dia 10mm along. Not something I'd do, after seeing a colleagues forks snap on some steps after he tried something along these lines many years ago.
10mm would probably make a bit of difference on some razor sharp race ht, but not a 5-6" FS bike. (IMO obviously).