I used to work for Brunel Metrology in Radstock - In my experience Torque Wrenches only go out of specification if they are preloaded (i.e you tighten a spring to set the torque) and this is left wound on.
Some cheap ones are often 5-10% off from new, but that's about the worst case scenario - that would probably fail the British Standard for accuracy, but is more than adequate in a mechanical maintenance setting even for tightening cylinder heads.