No. It's a myth that it causes arthritis. There is no proof. The only thing that has been shown is it stretches out the ligaments in your fingers. Read http://health.howstuffworks.com/question437.htm to learn more.
Which in a way answers his question that it is still bad "however, habitual knuckle poppers did show signs of other types of damage, including soft tissue damage to the joint capsule and a decrease in grip strength"
Not all the time -.- as Solus said, it does harm soft tissues around that area, and with that in mind, it can potentially harm veins? ahh whatever. It feels good.
Not all parts. Mainly the ligaments, joins etc as that is the part that stretches when you pop your knuckles
I would say cracking fingers and popping knuckles, legs, toes is most likely not beneficial to your well being.
