Grounding to the starter or anywhere on the engine block is better than the frame. Reason: the frame usually does not have a metal to metal hook up to the engine, because of the rubber engine mounts. Thus, a ground on the frame does not necassarily mean a good ground for the engine. Your engine needs a good spark, hence a good ground to get that spark. Generally, its good to have the engine and the frame gounded together with seperate ground wires back to the battery so a good ground loop is made.
