Please anyone reading this don't wear headphones, you've removed one of the most useful senses for knowing where there's a potential problem. I'd love to cycle to work, the only problem is it's a 160 mile round trip, so it's not too sustainable. I've thought about cycling the journey from my house to the train station but then I've got the additional problem of dealing with a bike on a busy train (two trains really),…
I very rarely ride on MUPs (Multiple Use Pathways - jogging trails) so I'm riding in the street with vehicles. When I'm not climbing all I hear is wind noise, and the sound of cars behind me. There is absolutely nothing that my sense of sound has done for me to keep me any safer. I can't tell which lane a car is in, or how close they are until I can see it. Headphone will at least cut down some of the wind noise, and in areas that have less tolerant people yelling out the window you may not hear them as easily - both increasing safety and enjoyment.
If you're really concerned about safety use a rear view mirror.