In most cases true, but not always. John Adams was a historical miniseries where the men actually had historical hair styles (and wore clothing that didn't cater to modern looks). (A Trailer to illustrate. Of course, it helped that their leading man was Paul Giamatti, who actually resembled historical Adams instead of being far better looking.) Charles II: The Power and the Passion also did well in this regard. I mean, they did use the out of postulating that beneath the periwigs, the men wore their natural hair short very short (both for hygeniec reasons and because a periwig is really heavy), and included various "private occasion" scenes of the guys sans their wigs, which helps with actor identification, but that's plausible enough.