Katya's post makes a very important point in that the corset actually is warmed by body heat and moulds and stretches. Thus when she advises that it be donned early, she is absolutely correct.
Our performers are advised to put them on at the half hour call, sit in them and do their make-up, have their wig put on etc. After about twenty minutes it is usual to re-lace the corset so that it can be made tighter and fit better. We also sometimes find that dresses with boned bodices, that is bodices that are not worn over a corset but instead have the bones built in to the foundation, often have to be taken in after a couple of rehearsals as they tend to stretch and become loose.
A corset that is too loosely laced will be just as uncomfortable as one that is too tightly laced.