Cleaned them all. mtb derailleur bits are such a faff to clean, too many little tight spots that collect shit.
If your pulling the derailleur off the bike there is a super simple way to clean them.
Grab a crock pot from a thrift store on the cheap, fill it with water and a little dish soap, not a lot, just a couple drips. If you have grease cutting dish soap use that, but anything will work. Turn the crock pot on low and drop the derailleur in.
The warm slightly soapy water will make grit and grime that's been caked on for years wipe right off.
I'm too cheap to buy a proper solvent tank for my garage(I've got one at work though if I REALLY need to use on). So I use the crockpot method for small parts. Some parts are too big for the crock pot, so I have a big 10 gallon storage container with concentrated simple green and a fish tank water circulator in it for big part.
Between the two, parts cleaning is super simple and easy.