Don't want to get into an A&S discussion here, especially with such a seasoned pro as chip, but as I mentioned earlier, there are no blind spots with a moveable head mounted mirror.

A head check is never a bad idea, just not really necessary even though I often do it myself strictly from habit.

We can aim that mirror absolutely anyplace we need to see. Now if the purpose of turning around is to show cagers you intend to move, that's a different story.

Here's an example...my latest commute is on a rural highway with a shoulder. I prefer not to ride on the shoulder if there is no traffic from behind so I ride in the lane and move over when necessary and then back to the lane when the traffic clears without ever having to turn around with a head check.

Of course the other advantage of being in the lane when cars approach is they immediately start to give me room thinking I may never move over but I always do.

In a car if you are moving laterally like that, a head check is absolutely necessary.