No. I'd be rather upset if my best friend dated one of my ex's. I'd be rather pissed off if ANY of my friends dated one of my ex's. I'd get over it eventually, but I'd be upset by it in the beginning.
Knowing that, I'd not do it to someone else. There are plenty of men in the world, I don't need someone else's man or ex.
Freshman year in college a good friend went to school in another city. She broke up with her high school boyfriend, who I happened to work with, before she left. She asked me to "watch out" for him though. He was lonely- I went out with him once and told her about it. Bad idea. It seems "watching out" for him did NOT include GOING out with him.
I would never risk the hurt feelings of a friend or our friendship over any man. Even if she said she didnt care...we women really know there would be a part of them deep down inside that it would hurt....
So men, dont ask out your ex's (or present g/f's) friends. Also, in my own humble opinion--it wouldnt say a heck of alot of your own personal character if you did.