I take occasional glances rearward, I generally can see the glare of the light.
It's a bit hard to afford the luxury of multiple lights running, particularly on Audaxes. I prefer to keep a spare for when the battery's running low.
Worrying about the rear doesn't do a lot about the ease with which I can take a wrong turn, though. The most spectacular of which was here. I thought something was wrong, so stopped, and even took a picture. But the road sign had said that the A40 was 2nd left, and this was 2nd left...
The gate being left open wasn't particularly helpful, either!
The road number was right, at least. And it wasn't for very long.