I've never heard that advice myself. I LOVE looking in the mirror tripping, especially on tryptamines because their visuals are much more 'flowing/morphing' than phenethylamines. Im always reminded that I'm an animal when I look in the mirror on mushrooms or miprocin. For some reason my perception of myself is animalistic. I theorize that this is bEcause I'm confronted by something super familiar, my face, but slightly altered, and thus alien.
I also find that my mouth is hugely exaggerated, being much larger than it ought to be. Especially if I smile I am astounded by how big my mouth is!