The accelerator I participated in had relatively open doors and investors who came to everything from office hours to pitch practice. We ended up taking money from one of the most involved investors first, (and left some who really wanted to participate out) but the group of investors who didn't get in on the deal said, "Well it's fair if they want to put that much time and effort into it; we're not willing to do that."
It is, however, a major signal; if the guys who know you intimately well don't want to invest, could cause other investors to be cautious/skeptical, which isn't fair to the companies that don't get an investment from the YC partners.