Nice post! Much needed!
But also make sure, when taking measurements you also take into account the shape of the body.
My two dolls have a sandglass figure. (like most do I imagine) Thus, if I looked at the smallest measure, a skirt for example would really be hard to put on once they pass the thighs for example. But they would be completely loose around her waist. So, I check every measure and take the largest one into account. If for example, it is tight around the thighs but loose around the waste, there are always belts to help tighten it around the waist.
Some stores do take the shape of the body into account, so make sure to take it into account when you can.
Also, reviews of the product itself might help you decide if you should take a measurement smaller or higher.