Clevite's Bill McKnight says that while a micrometer may be used to measure the rod-bolt length, the most accurate method is to use a specialty fixture that is outfitted with a dial indicator. This is referred to as a rod-bolt stretch gauge and is available from several specialty tool sources.
Connecting rod bolts should be regarded as high-tensile springs. The bolt must be stretched short of its yield point in order to achieve accurate, and, most importantly, repeatable, clamping of the rod cap to the rod. Improper or unequal bolt clamping force can easily result in an out-of-round rod bore.
Stock, or production, rod bolts typically offer a tensile strength of approximately 150,000 to 160,000 psi. However, due to variances in bolt production, tolerances can be quite extreme, with peak bolt stretch travel occurring anywhere from, say .003 to .006 inch. If you use only a torque value in the attempt to achieve bolt stretch, you run the risk of achieving unequal rod bolt clamping loads. High-performance rod bolts are manufactured to meet much tighter tensile strength tolerances.