Grass Fed beef cooks more quickly than grain-fed. Due to the difference in the composition of the fats, fat in grass-fed beef melts at approximately 10 degrees lower temperature than fat in grain-fed beef.
Not allowing for this difference can easily result in over-cooking, and a dry, unappealing result. One of the main reasons we have steaks cut thick is to reduce the chances of this happening; it builds in a margin of error. As a note: if you are a fan of 'well-done', and prefer your beef cooked to a solid shade of warm grey - save yourself the time & money; you're better off buying an organic grain-fed cut from the supermarket. At that stage of cooking, any grass-fed beef will be dry, probably tough, and nearly flavorless.
The most important kitchen/grill utensil you can have is an accurate digital meat thremometer. Meats should be removed from the heat source when its internal temperature is 10 degrees lower than the stage of doneness you favor. 'Tent'the meat loosely with foil, and allow it to rest for five to eight minutes per pound before serving.
During this rest period, the heat of the outer surface will transfer throughout the meat, essentially cooking the interior with residual heat. This is going to happen whether you allow for it or not; the perfectly 'medium' steak you place on the plate straight from the grill will be a well-done steak when you cut it five minutes later. By building the heat-transfer process into your cooking practices, you both save energy and produce a better product.