Bill Tindall, E.Tn.
If doing the drying to constant weight, the NHLA reference method, it is just as easy to do it right as not. Dry in an oven to constant weight at 220 F.

There are two problems using a microwave oven to do this measurement.

If there is a deposit in the wood that preferentially absorbs microwave radiation it will pyrolyze, fill the kitchen with smoke and you will be banned from doing experiments in this device. I don't recall why I was microwaving this piece but you can see it was a bad idea.

Second, it takes more than a few minutes to diffuse moisture from the center of a piece of wood. Drying to constant weight becomes tedious with the constant attention microwave drying involves. However, it is a good way to get a quick approximate check of the moisture content to see if it too high, if it doesn't fill the kitchen with smoke.