Searched my freshly arrived 2011 FAR/AIM. No mention of shoes. anywhere.
My seaplane instructor told me to always have my shoes off when beaching the airplane in case I have to jump out to secure the airplane in a hurry.
There might be a reason for wearing them on the ground. If the airplane were to break or an engine were to explode and an evac was required, it might save feet from being cut on shrapnel. Rare, but then, you never know when the underwear might explode.