Some populations have more parasites than other. Sashimi is cut in fine translucent lames in part for easing to spot the parasites (or more precisely to verify the absence of parasites).

But if you're using the freezing method, the parasites will still be there but be dead. Using modern methods, it seems any species of salmon is safe to use for sashimi. Some are just more palatable than others.

If the parasites are visible my understanding is that is not sashimi grade meat and would be smoked or processed in several ways instead. I assume that Japanese would assure to remove any undesirable aspect in the meat before to make the fish and serve it just because would ruin aesthetics. I could be wrong.

Just for aesthetics, maybe in Japan. In the US, there's no standard for sashimi grade fish. So there's no real difference between most fish marked sashimi grade vs most IQF.