Working in a hospital maybe has me slightly over cautious, because I have seen the super funky bacteria and yeast (yuck). I assume their is a plethora of organisms floating around or tracked in on the bottoms of shoes. The point is who knows, so I want to kill the unknowns. My other assumption is that the exterior layer of cultures are more superficial due to the lack of sufficient osmotic pressure to penetrate the wood, but the internal pressure is strong enough to push deep into the grains.
Fortunately, I have three weeks before I have to pull the trigger. I have thought of the Star San soak and the short 2min boil. Hitting the exterior with a course grit sand paper makes sense too.