I like when people seeking feedback mention they're a YC reject (if not in the headline in the accompanying text). They're demonstrating their relentlessness and it satisfies my curiousity to see what sort of teams/projects weren't selected.
To me, it's just as appropriate to mention as the fact that other companies are "(YC'08)" or whatever.
Now I'm not exactly a suave Casanova, but I have learned that introducing myself as "Hiya, I'm Patrick and I just got shot down by that girl over yonder. What's your name?" is not a way to endear myself to people. I think this insight is probably transferable to business.
Your application is what it has accomplished, not what it hasn't. Highlight accordingly.
True, and I wouldn't recommend leading with "YC reject" if you're courting an investor or customer.
But teams asking for help/feedback from peers, here, benefit by demonstrating earnestness and tenacity. They don't have to impress by selective disclosure. Acknowledging, without obsessing over, the prior YC rejection at the same time as releasing other positive news heightens the emotional impact of that news.
Founding myths often highlight the many rejections that had to be overcome. For a team that's really on a mission, it's never too early to start sending the message: nothing stops us.
I think working applications and paying customers demonstrate earnestness and tenacity more effectively than tales of the string of failures you've suffered to this point.
But teams asking for help/feedback from peers, here, benefit by demonstrating earnestness and tenacity.
More to the point, I think, is that they benefit by fully disclosing their situation. Disclosing that you are not YC funded (either through rejection or simple disinterest in applying,) is helpful when one is asking for advice.
It might not be something that you want to include in the headline, but it's certainly worth mentioning, especially if you are looking for VC or financial advice.
To me, it's just as appropriate to mention as the fact that other companies are "(YC'08)" or whatever.