It's definitely rewarding to be the first of your friends to know about a great band. But there are so many bands out there, even people who pay close attention to music are going to miss many more than they discover. Unfortunately the hipster-wannabe-music-loving culture often fails to be humble when faced with the prospect that someone else scooped them on fresh music.
As indicated by the below video from the Jimmy Kimmel show, this failure to be honest can end up making people look like idiots. It seems that being in the know about music is much more important than just knowing what you know and enjoying it.
The best approach when presented with a band you've never heard of before is to be humble... admit you don't know them... and then if they are good, be honest when you share their music that you just heard of them, even if their album came out years ago. If you don't, you might end up looking like these people: