Should you use a weight belt when doing exercises like squats and deadlift? The answer is generally no.
Unless you are a powerlifter using super heavy weights, you shouldn't use a weight belt. You need your body to develop core strength and stabilization without the help of a weight belt. If this means lifting a little lighter load to develop core strength then that's okay! Don't let your ego lead to low back injuries!
If you are recovering from a low back injury, a weight belt could come in handy until you recover. In this case, tighten your belt during the lift and loosen it between sets. Research shows that your blood pressure can significantly elevate during the lift while wearing a belt.
As a general rule, you want your body to develop strength under loads that it can handle. Sacrifice lifting less weight on exercises like squats, deadlift, good mornings and standing shoulder press to naturally strengthen and stabilize your core and other musles.