It depends on how bad your form breaks down. I have a certain weight in the squat and pull that I put on a belt regardless of how I feel that day.
My best squat is 405, and I put on a belt for anything 315 and up. My best pull is 500, and I put on a belt for anything 365 and up. I don't base it on the numbers themselves, it's just a "feel" thing.
You have to find your own points for each lift where you feel that bit of form breakdown and need some support from the belt.
Jim always says there is no medal awarded for being beltless. Be safe with it and live to train another day.