It shouldn't be too suprising to learn that in Japan, soy sauce is used with beef. Afterall, soy sauce is an integral, foundational component of "Washoku," or traditional Japanese cuisine.
If you find yourself in Japan's smallest province of Kagawa and venture the 1.5 hour ferry ride to the tiny cow-shaped island of Shodoshima, you will find olive tree groves, artisinal olive oil producers and a handful of Olive-fed Wagyu cattle producers. And if you stay long enough, you might also find Yamaroku Soy Sauce, one of the most interesting soy sauce makers in the world.
Most soy sauce is produced quickly for a mass market using giant stainless steel fermentors in a matter of days. However, Yamaroku Soy Sauce is working to preserve what's fast becoming a lost art, using original 850-year-old wooden barrels to ferment soy sauce for 2 to 4 years (depending on the variety), a process which leads to subtle flavors that can't be replicated any other way.
Next time you have Wagyu, I recommend trying it with a little high quality soy sauce and a pinch of fresh grated wasabi. Bon apetite!