Not always. There are times to put your head down and build (depending on team composition and what your business is doing, of course)

It totally depends on the product, but from the YC PoV, you have 3 months to build something that dazzles investors. Generally, YC discourages hunting for networking/bizdev/funding deals because you have EXACTLY ONE SHOT to impress a room full of the most important early stage investors on the planet.