C-clamps can make nasty dents that are tough to sand out. Pads or blocks will prevent this, but they're often awkward to hold in place while you tighten the clamps. I make pads that don't require extra hands.
I bought a roll of 1-in.-wide adhesive-backed magnetic strip from a craft store and cut off a number of long lengths. I adhered these strips to 1-1/4-in.-wide pieces of 1/4-in. plywood. Using a miter saw, I cut these pieces into shorter lengths as needed to fit the C-clamp jaws. I store my magnetic pads on a steel strip that I screwed to the end of the bench. The pads are ready to go whenever I need them.