the best way to paint????

is it bad to just sand down the stock paint on a vehicle, then prime and paint it with new paint. i see on TV cars getting "media blasted" or the paint being stripped with a stripper. is that really necessary? eventually i am going to be painting my truck so i was wondering? also, if it's ok to paint over the stock shit, how many coats would be too many? (if the trucks had a paint job before).