I never heard NOT to look into a mirror while tripping. In fact when I was 17-21 I would be tripping in my house and talking/bugging my parents and they would always tell me to go look into a mirror and get side tracked staring at myself.
I had a lot of fun looking at myself in the mirror.
The only time I got kind of freaked out looking at myself was when I was tripping on lsd. I was wandering through my living room and stopped to look at a baby picture(hanging up on the wall) of me at around 2 years old. At first I started to have a wonderful inner monologue about how if baby me could see how he grew up into a nice,funny over all good dude(older me) he`d(baby me that is) would be super happy. So I was pretty happy with my thoughts.....until the picture of baby Me started to morph into a creepy withered old man that started to turn and look at me.
So I ran into my room to listen to music.
Anyway,I never had a crappy time looking into a mirror.
Also I totally agree with Bagseed on the positive experience with the mirror. I had pretty much the same thing.