Yes, in the interest of aesthetics and proper acoustics the correct answer should be nothing.
Sometimes space becomes a factor and that area under the machine is too tempting not to utilize. I currently have a pedal tractor and camper oven stashed under the machines at home (and some dust bunnies/the occasional dog or cat under a couple others).
The shop game has a transmission jack underneath it. . . sort of.
Do you have anything unusual hidden under your machine?