General Motors has recommended for years that owners flush the underbody of their vehicles at least once a year to remove salt deposits, mud, dirt and other accumulated crud that could cause corrosion in the long term. Other manufacturers may recommend that as well, and even where it isn't recommended, it is a good idea for those who engage in off-road driving or frequently travel unpaved roads.
GM doesn't make a big deal about underbody flushing, advising that owners should use plain water to clean the underside of vehicles so salt and other debris doesn't take up permanent residence. This can be done at a car wash (which sometimes offer underbody cleaning as an extra-cost service), with a garden hose in your driveway, or if you're the type of owner for whom maintenance expense knows no bounds, you could take it to a dealer. We can't help but wonder, though, how much cleaner the underside of your vehicle would be as a result of having a dealer do it (and how much would it cost)?
Perhaps having your vehicle washed on a regular basis and making sure some water is squirted where the sun doesn't shine is all you need to keep the underbody clean. Just don't get carried away, because you could do too much of a good thing. Though GM advises having an underbody flush annually, it also cautions, "Do not directly power wash the transfer case output seals" on all- and four-wheel-drive models. High-pressure water could damage the seals and contaminate the transfer case fluid, which would then need to be replaced.