I wouldn't "completely" disassemble the derailleurs or brakes. You can remove the inner cage plate and pulleys from the rear derailleur for cleaning and lubing but do NOT take the parallelogram or the other pivots apart. You can effectively clean these parts in a solvent like OMS or Kerosene using an old toothbrush to get into the various recesses. Blow or shake off the excess solvent, let them dry and relube. Also, don't remove the headset cups or crown race unless they are to be replaced with new ones.

Edited to correct the recommendation on derailleur disassembly!