How accurate does a torque wrench have to be?

5% is acceptable for a clicker. Most that I send in for calibration come back within 2-3%. Beam style wrenches are only as accurate as the user. The scale marks are quite close together and are hard to read consistantly. The dial wrenches are just about the same. The new digital wrenches are quite accurate as long as they are calibrated correctly. (the same as all torque wrenches)
I truly believe that as long as a torque wrench is reasonably within spec (within 5%) the consistency of the torque is far more important than the ultimate accuracy. A head bolt doesn't fail if it is torqued 1 or 2 ft/lb tighter than spec but if it is a ways out of calibration you may be in trouble. All the new torque specs are torque to yield which purposely stretches the bolt to the proper torque. Get your torque wrench calibrated, back the adjustment down when storing it if it is a micrometer style, and use it properly. Don't forget to lightly lube the fasteners before tightening. (don't assemble dry and don't drench the bolts either) That will give a more accurate torque. tmm