My bike is equiped with lights, reflectors, bags, but not a mirror.
I prefer to keep my eyes on the road, and use my peripheral vision and hearing. I dont seem to have any problem detecting traffic comming from behind.

A mirror forces you to focus your vision in a small area, and you can lose peripheral vision for a moment. Since danger usually comes from the front and sides rather than behind, I dont like to take my eye off the road.

If you want to use a mirror, thats fine, but check how it affects your whole looking process. I tend to use a scanning mode, searching the road surface, the traffic several vehicles in front of me, the edges of the road, any upcoming junctions, and and behind me, in a continuous process.