People also say you should de-rate as a thread wears or if a bolt is re-used..
Personally I don't think bike engines are that stressed, or tolerances are that close to the max., so I tend not to derate. I clean & degrease the bolts where practical but often thread lock... except for engine casing bolts as their thread are made of cheese so I live in fear of them.
I actually don't use a torque wrench for small bolts, I've just developed a feel for them - using 3 fingers and a small 1/4" rachet.
I just tighten until it goes loose then back up 1/4 of a turn.