The workshop conditions that the instrument was built play a big part. Having said the above it must also be kept in mind that a lower temperature in the practice area when it is very cold outside makes keeping the humidity up easier. The less the temperature gradient from the outside the easier it will be to maintain the humidity. Performers that take their harpsichords out to play in dry churches or music schools do face the delema of what to do. One can also use a bug sprayer filled with water to raise the humidity just before playing.
Carpets are a good place to spray.
David talks about too much humidity which only really occurs in the far south. But the problems of tuning and regulation are far greater in the drier climates. The only real solution to high humidity is air conditioning. Damp chasers are best avoided in the northern climates.