If you are talking about a kaiseki dinner, yes sashimi will come before sushi, but you'll get a couple of pieces of each, as part of a multi-course meal. If I go to a sushi bar, I'm there to eat sushi (or sashimi, or both). It is not a "filler", it is the meal. Sometimes I'll have sashimi and then sushi (and not the other way around), but the main attraction is the sushi. Sometimes I'll have just sashimi, most of the times just sushi. Mrs. Pickles usually only eats sashimi, since she likes to take small bites of her sushi and that's a no-no in fine establishments.

Otherwise I don't get your question.