Sometimes, you can simply bolt the new TB on and your idle will be correct.
But most of the time, you will have to calibrate the throttle body to get the idle set correctly. Instructions on how to do this SHOULD come with the throttle body. In short, for our trucks you adjust the throttle set screw (attached to the tb butterfly) until the truck can idle at approx. 500 rpm with the IAC unplugged.
If this is not done, the vehicle can idle too fast or too slow, causing higher fuel consumption, odd acceleration/deceleration, and probably setting off a CEL light for TPS sensor voltage out-of-range. Putting it another simple way, your truck would run poorly, and could even be dangerous if you idle at 1000 rpm, causing you to have to keep the brake mashed down to keep your truck from surging forward at 25 mph.