Esposing the enamel to acid is always bad. I have had 3 dentists and all warned me about things like biting into lemons and things like that because if the damage it can do to your teeth. For example, bulimics often experince tooth decay as a result of the stomach acid on their teeth destroying the enamel. Obviously lemon juice is not nearly as acidic as stomach acid, but it can still cause damage, especially when exposed repeatedly overtime. Trust me, I know firsthand!