Two things in BobG's defense. First, you can get the masking tape right on the rim when you start, then spin the wheel. If a gap opens up, or the rim shreds the tape, it's not true.
Second, I've rarely had an issue with radial true after the wheel was built. I guess if you broke 3-4 spokes right next to each other, for instance if a stick threw the derailer into the wheel, that could be a problem. But a single loose or broken spoke can usually be fixed without problems.