If you're unsure about whether or not you should wash the undercarriage of your car, the answer is most likely yes. Here's why:
The undercarriage of your car is exposed to a lot of dirt, grime, and salt (if you live in an area with winter weather). This can lead to rust and other corrosion if left unchecked. Washing the undercarriage helps remove these corrosive materials and keep your car looking (and functioning) its best.
Of course, you don't need to wash the undercarriage every time you wash your car. But if it's been awhile since it was last cleaned, or if you notice any build-up on the underside of your vehicle, it's probably time for a good cleaning.