NO NO NO! I've made this mistake. I need to know about exes in the BEGINNING of the relationship. I know that many will say it's taboo...but I've dated some guys who played that "I just don't want to talk about it right now" card, and later when they fessed up, I felt duped.
The one that ended earlier this year, he finally opened up more about his last relationship. I learned about how manipulating he was, and that he would constantly break up with his last girlfriend as a way to control her. The last time he broke up with her, she didn't take him back. He was still in love with her while he was dating me, he dumped me because he figured he might still have a chance with her, she told him she didn't want anything to do with him. He's a mess now.
I see crack in his future.
My last relationship, I had to beg the guy to tell me why they broke up. He finally fessed up that it was because his parents were racist (he is white). I felt like I should have known about his racist parents from the beginning. He and his father almost got into a physical fight when he announced that he was going to marry his last girl. So he felt torn between what to do, his last girlfriend took this as a sign that he wasn't willing to stand up for her, so she broke up with him. This was a sign that right now, his family situation can complicate things. No sir, this is the 21st century, I ain't dealing with that!
Sorry for going on a tangent, but I just wanted to show examples of how important it is to know these things. You don't have to know all the details, but enough to figure out what sort of obstacles your own relationship might face.