Some time ago I heard that if by picking up the front wheel about an inch off the ground and dropping it resulted in a vibration instead of a solid thump, it was time to maintain/clean up the headset bearings, I can't vouch for this since i've never re greased mine. As for cranks and BB, since I installed my sealed BB I haven't re greased BB or cranks. I would however recommend more regular cleanings if you often ride in the rain or other harsh conditions. A bike is no different from any other machine, preventative maintenance is key.