Technically, it's not important unless there is a specific need to do so. For example, if you have a food allergy or you need to consume a certain amount of calories then you should definitely read the labels. This is a great question. Unfortunately, labels are supposed to be easy to read but they can be quite ambiguous. For example, manufacturers may put a "fat free" label on candy or cookies. While this may be true, such foods have the same, if not more calories than the regular version. A typical shopper does not have sufficient knowledge of how to read labels and some of the complicated ingredients. For the record, it would be impossible to fully understand what labels mean without a class or two on nutrition. The following explains the food labeling guidelines which may include terms such as "low calorie," "low sodium," and "low fat."
http://www.fda.gov/Food/GuidanceComplia ... 064911.htmA label with "caffeine free" can have up to 3% of it's caffeine content (97% removed). As long as you are not overeating, you should be okay. If possible, follow the food plate (formerly the food pyramid) as closely as possible.
http://www.choosemyplate.gov/