Few divers pay any attention to the type of knobs found on manifolds. If they did, all manifolds would have rubber knobs. They are durable, shock absorbent, shatterproof and easy to turn. Divers should be aware that rubber knobs could turn inadvertently if rubbed along an overhead.
To avoid "roll off" of the gas supply, the alert diver merely needs to check the valve following any contact with the overhead (a wise habit to develop regardless of the knobs used). Plastic knobs do slightly reduce "roll off" but are dangerous because they can shatter on impact, leaving you with nothing to turn on or off. The metal knobs can’t shatter and are less likely to roll but can bend or shear off upon impact.