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I usually just brush them down when they are hot with a wire brush. You can put them in a self cleaning oven but be prepared to open the house cause it does get smokey!
wire brush and onion for me, at three hundred degrees its one of the best aromas you'll have .I also place the used portion of the onion close to the coals in the firebox for extra flavor.
I put my grates on my gas grill cover with aluminum foil and turn it up on high and just let them cook. Wire brush them and give them a good coating of vegitable oil. It works great and keeps the smell out of the house
If yours are the nickel-plated kinda lightweight type like mine, they will corrode pretty easy and I wouldn't hit them with the dishwasher.
I soaked mine edgewise in the sink overnight with just a little dish soap and a few small rusty-colored spots appeared.
Got them clean though and the spots rubbed off.
I try to remember to spray the grates with Pam first, that helps cleanup later.
If you have the time and are lazy like me spray them with oven cleaner, place them in large garbage bag, tie bag closed and let set overnight and part of the next day. Take out,light brush and spray with water hose. Good to go.