I've never read that you shouldn't. Note that if you believe everything you read, you'll be in a constant state of contradiction.

I love to have my big girls and boy hang out on my neck because then i can get other stuff done and they can come along. I once had to get somewhere with one (mind you i bike everywhere i go) and she's pissy when you put her in a bag, so i let her ride on my neck. She was so completely chilled out you would swear she didn't even realize we were going any faster than i do when i wander around the house *shrug* (yes i had a bag for her in case she misbehaved, but she never did)