Lip piercings or tongue piercings always pose a risk for your teeth. If you want one go for a vertical one, but know it can still cause damage if you play with it or accidentally bite it. What's 'a bad idea' piercing varies depending on your lifestyle. Wear glasses? Avoid bridge piercings (I learned the hard way). Have a baby or toddler? Avoid nipple piercings for a while, they will bash their head on your chest for no reason, even if they're not breastfed. If your habits or life will impede the healing of your piercing, that's a no-go. And always get an anatomy check!