I had a handlebar mirror on my commuting bike (flat bars), but I found two problems with it: it widened my bars to the point of making it difficult to negotiate tight traffic, which I must do in the streets of DC, and it kindof got me off balance. I decided that I was safer without a mirror, that I am better off paying attention to what is going on in front of me.
I can't use one of those helmet mirrors because I am blind in my left eye and the mirror wouldn't do much good on the right side! Plus, it would block my good eye, and then we'd all be in trouble.
But you'll have to decide for yourself what is just right for you.