How accurate does a torque wrench have to be?

Let me tell you my horror story, brand new craftsman click wrench, 30 years ago, building a 440 and I snapped one of the short head bolts on the bottom of the head rebuilding a 67 Magnum. Took it to work and our calibration lab, thing was 30 lbs short, ie. 70 on dial was 100! So they melted the lead from the end, calibrated the spring and put it back together. Now I do my first steps, say 20 and then double check with my beam wrench to check the cal. If the wrench doesn't come with a calibration certificate, it's not calibrated. Period.