The most important time for a mirror is when you encounter an unexpected gauntlet. Suppose as I approach a driver who's pulled over onto the narrow shoulder to talk on the phone, I see a car coming the other way; I gauge my own speed, I gauge the speed of the oncoming car, and I consider whether I'll pass the obstacle before or after the oncoming car. Now is not the time to look behind me
You must ride faster than me. When I see anything in the distance that might block my riding I look behind me. I'll do that for a car in the shoulder or at an intersection if I see someone pulling up to the stop sign. If it's clear I'll move over into the lane for better visibility. If you keep your vision up you won't be surprised very often by obstacles in front of you.