It's always a good idea to invest in a shower liner, particularly if your shower curtain is made from an absorbable fabric like cotton. Plus, it's an extra layer of protection from keeping your bathroom flood-free while you lather up, in turn preventing the buildup of mildew. No thank you! However, if you've snagged a plastic or vinyl curtain, a liner might not be necessary. When in doubt, check the label or packaging for tips.