When using real wasabi (i.e. freshly graded wasabi radish), never mix it into te ranu rei, as it would lose the delicate flavor. However, it’s rare freshly graded wasabi radish served unless you go to high-end restaurants. So let’s talk about the benefits of not mixing “any” wasabi into soy sauce.
For sashimi, depends on the type of fish, amount of wasabi to complement the tastes would differ. We may want more wasabi for tuna, and less for snapper, for example. Also, when I eat daikon radish commonly served with sashimi, I want no wasabi at all. So keeping the soy sauce in the small dish clear is easier to vary the wasabi amounts. So I usually put on a little amount of wasabi on one end for each piece of fish, and dip the other end to soy sauce.