Under ideal conditions, the body will regulate itself. However, this requires eating natural foods most of the time, which most people don't. If your diet consists largely of packaged foods, snacks and other junk food, your digestive system will go out of whack. In this case you would do well to undertake a 'cleansing', as long as it consisted of all natural ingredients, and worked slowly (at least 2 weeks).
Once you get back to eating natural foods, and an appropriate level of exercise, you shouldn't really need a cleansing more than once a year, if that. A couple of teaspoons of psyllium husk in a glass of orange juice each morning, followed by a glass of pure water, each morning, will help your digestion.
The other reason for a cleansing, is to remove undesirable parasites from your system, but that's another story. You need a cleansing product that will specifically do that.