Are there general rules of thumb for when to marinade a steak vs seasoning with a little salt before cooking? Should marinades be reserved for grades select and choice or can they be used on prime grade too (eg. a ribeye or NY strip rated USDA prime)? Is it ever recommended to marinade a tender cut like prime rib? Does cooking method (grilling vs broiling) influence whether a marinade should be used?
Marination is used to add flavor and seasoning (if the marinade contains salt). Some marinades, which contain active enzymes (as from fresh pineapple or papaya) or acid (as from yogurt, lemon juice, or vinegar) can soften meat as well (although they should not used for too long, as they will turn it to an unpalatable mushy texture).
Contrary to myth, marination does not penetrate more than a few millimeters into meat, so it does not flavor the interior, although salt may penetrate further if given time.
Does the grade matter?
No, the grade of the beef is not relevant to whether you choose to use a marinade. Higher grades of beef simply have more internal marbling.
The real question is whether you wish to compliment the meat with the flavors in the marinade. Dry rubs are another technique for adding flavor. Finally, simply salting is just for seasoning, but doesn't add any distinctive flavor.
Broiling versus grilling
Broiling and grilling are essentially the same technique. Both are primarily radiant heat methods: the infrared from the fire or element is what cooks the food. With grilling, the fire is under the food; with broiling, it is above. There are even restaurant appliances called salamanders which have elements both above and below.
If there is any practical difference, it is easier to get more intense heat on an outdoor grill than it is with an indoor oven.
None of this affects whether you choose to use a marinade on a particular piece of meat or not.