I clean away anything that could make something not work as well, but mud on the downtube doesn't really bother me. Makes the bike look more manly. I lube my chain before every ride, however, and after the muddy ones I hose off all moving parts and lube what needs it.
I got a deal with my local bike shop, getting a brand spankin new xt, grunge guard, hanger, etc for about $70. Better than if I ordered online and then got it installed. And way less than if I tried to install it on my own. I'm not that comfortable fixing my bike. I can tune it, but this is a little bit more shady of an operation. I think the derailleur is an 06 model. It has the new swoopy cage design and new body design. I'll post a pic of it when I get it, and one of the whole bike for all of you uneducated fools who don't know what a 29er is like.
peace
harris