I have the same grill. The reason that you oil it is for the cooking process. As long as you don't have it exposed to the elements and use it often enough (using the oiling method for cooking), I don't anticipate any rust. I have had mine for 2 years in the Northeast without a cover, and have had no signs of rust on the grill yet. I ensure that when the rain is here or approaching, I close the top vents to prefent water from getting inside. I don't think that a nice heavy coat of oil at the end of your cooking season (if there is such a thing), as well as getting a cover for it would hurt. Make sure that when the weather is nice to remove the grill cover however. If it is very humid out, the cover will trap the water vapors inside and actually accelerate the erosion process. Don't forget to season your grill and then reseason once or twice every year. Hope you enjoy it as much as I do. B.T.W. my gas grill has loads of rust on it.
Chris