I remember working with Shell. They have three volumes of rules on how you can treat the pectern (the Shell logo). No colour changes, proportion changes, definite rules about handling it on different backgrounds, about it's use if different contexts, about use with text... On and on.
Ironically if DC had been like that they'd actually have a far weaker brand.
"the pecten symbol currently in use worldwide was designed in 1971 by loewy. the design and testing process completed by loewy's firm took more than four years. one of the tests involved hanging various prototype pectens on poles where they could be viewed by drivers passing on a nearby british motorway. drivers were later contacted for their opinions on the prototypes."
Improbably but true. I'd link to their website but they've turned the whole thing into Flash so I can't. Their design has clearly been going downhill since 1969....
(Or do you only mean third parties working directly on your behalf? I suppose in that case guidelines would be important.)