Technically a wrench should be recalibrated regularly. There are services that do that, or you can build a calibration rig pretty easily. Adjusting the wrench can be tricky, though.

Personally, I haven't calibrated my wrenches in a long long time. To justify my laziness I reason that even if I'm off by 10%, it's still a hell of a lot closer than guessing. (And within the spec range.)