DO NOT wash your weber grill parts! They will rust! Allow everything to burn and then scrape it with a wire brush. Don't worry about scratches. You only need to scrape the grill grates with each use. What I do is after I am done cooking, shut it off and let it cool. The NEXT time I grill, preheat at the highest setting for 10 minutes. THEN, scrape the heck out of the grates. Then grill. You can use a cooking spray, but I don't recommend it. Rather, lightly spray your food, not the grates. I use an Olive Oil spray (JUST olive oil, no chemicals).
The very best way to keep your Weber in good shape is to buy a fabric cover and use it every day. When it rains, water gets into the grill and will eventually rust the grates and flavorizer bars. I just had to replace both these parts in a less than 3 year old grill because I wasn't good about covering it.
The warranty does not apply to grill grates or flavorizer bars, and they WILL rust out if they get wet, or if you use a lot of marinades or sauces that are high in acid content. If you have SS grates, you can clean them with water, no problem.
I actually advise to replace the standard grill grates with stainless steel ones (unless yours already came with SS grates). The porcelain coated enamel doesn't hold up very long.
Do NOT worry about any build up on the inside of the grill hood. That is normal and will flake off on its own. Once a month, take the grill grates off and scrape down the flavorizer bars, and check for gunk in the v shaped tray underneath. That stuff, if it builds up long enough, WILL CATCH FIRE. If your grill ever catches fire along the bottom, throw some baking soda on it. Will put it right out. This can be avoided by keeping the drip pans clean.