It depends on a couple of things, such as the condition of the paint (if it is flaking, or peeling off) and how many times the car has been painted before. If the paint on the car isnt in good condition and is not sticking to the car, then what you paint over it probably wont stick either. if it is in decent condition, and adhering to the car, then a simple sanding would be best. Cracks in the urethane bumper paint, usually will go all the way down to the plastic, so getting the paint off there will be needed, or you will have the cracks re-appear after paint. If the car has been painted several times, or has thick paint on it, if you repaint over this it will chip easier, and the chips will usually be larger and deeper. unless yuo are a good bodyman the stripping is usually left up to professionals, or those that have upper level experience, as well as a good work ethic. Stripping isnt always the easy way out, as it can uncover and ruin unknown bodywork that will all have to be redone.