What book was that? The Big Book of Making Life Harder for Nursing Moms?
This is an overcautious suggestion, appropriate only for babies with known enamel defects or evidence of decay. The sugars in breastmilk are not the ones associated with tooth decay, and because babies must be actively swallowing in order to keep milk flowing, breastmilk does not pool in the baby's mouth at night. That being said, it's a great idea to wipe down your baby's teeth once a day, and before bedtime is a nice time to slip that practice in.