A mirror is a good thing, though yes you shouldn't rely entirely on them. I've found that practicing without a mirror does a lot to make sure I'm not overly focusing on my hands, but you do occasionally need that other perspective. You simply can't see from that angle if you're not looking into a mirror, or using a camera.
There have been plenty of times where I thought I had an angle perfectly only to find out (when I did in a mirror or on camera) that I was totally wrong. The opposite has happened a lot as well, where I thought I was way off and I actually had it right.