Wiki says: "Shoes but no socks. Barefoot / stockings / thighhighs don't count - only shoes worn over bare skin."
And what about something like this?
As you can see, she's wearing very high boots, so no lower portion of legs is visible. But after 13th episode of Mayo Chiki! now it's officially confirmed that she's not wearing socks with them. I don't believe she might took them off with her boots during the visit, so it's safe to assume she wasn't wearing any from the beginning. The "no_socks" tag would be perfect for this occasion. What do you think? Can we use it here, or should it be used only when "no socks" is clearly visible at the picture?