I'll ride with my iPod and SHURE E3Cs (which are isolating) pretty much all the time. The only exception to this is riding at night on country roads, because they have little, if any lighting. Really, if the car is gonna hit you, its gonna hit you regardless of whether you can hear it or not. If you use sound as an indication of approaching cars, then that's just silly: its far safer and more reliable to shoulder check every minute or so.