I commute through downtown Seattle every day & wear headphones. It's not very evident when you're in a car, but the wind in your ears is very loud. Loud enough that you can't use your ears to hear the cars behind as they approach. I've found a mirror to be a much more effective way to be aware of approaching traffic from behind.

The headphones I wear are not just some ear-buds -- they are seated in my ear & block out most ambient noise. I wear them to preserve my hearing.

Personally, I enjoy my rides more not having the sound of thundering traffic rolling past over the roar of wind rushing past my ears. I'm quite aware of the traffic around; I can still hear them as much as I would if I didn't have the headphones in. But it's much less intimidating (not to mention the music makes things even more enjoyable).