Torque Tolerance

[JFP in PA]

1. Buy the best torque wrench you can afford. Instead of looking at it as an expense, look at it as an investment in correctly maintaining your car.
2. Every fastener in the car has a torque range; they are published in the OEM service manuals, which again should be viewed as an investment, not an expense.

[bwdz]

It's really not that complicated. You will never achieve 100% accuracy when torqueing bolts or nuts. There are too many variables even in the production of the bolt or nut itself. The slightest bit of dirt in the threads will affect it, the slightest wear on the threads will affect it, the temperature will affect it, the stretch of your socket will affect it, how many times the bolt has been torqued and stretched and the threads stretched will affect it. The correct torque rating is a baseline and gives room for some tolerance, no need to get crazy complicated about it.
Heck, spend a $1000 on a wrench and buy new bolts every time and rechase and clean the threads every time and oil the threads before tightening every time and tell me how much more reliability you get? The answer is probably none over a common sense approach. I have never torqued an oil drain bolt, nice and snug is about it, all of us who wrench know not to crank on them and I have yet to strip one or have one leak.
For internal engine work I do follow all the specs and procedures as carefully as possible but knowing that it can never be 100%.
If you have not wrenched much and never developed that feel in your hands then certainly use a torque wrench but don't sweat being that exact. It is most important that you always clean and take care of the threads and use lubricants or anti seize where it is called for (especially in water passages or in different metallurgies )