You should caulk any and all corners and joints in your home, especially where two different materials meet. That means around trim, baseboards, doors, and windows as well as in all corners and joints in your shower and bathtub areas, around sinks, at the bottom of backsplashes, and on the edge of countertops. Basically any time you need to fill a gap or crack for a finished look or to waterproof, use caulk!