Well, wherever you attach them, I think mirrors are a great safety element when biking. Sometimes I find myself trying to glance at my helmet mirror when I'm NOT biking but just walking down the street....to funny!
I think I'm going to get some clip-on sunglasses for this summer and just clip them onto my prescription glasses. Next time I get my prescription changed, I'm going to spring for an extra pair of glasses with tint that darkens in the sun.