I LOVE this question! I never thought of some stranger would walk up to me and play with my skirts, figuring out how they were supported, but I really have had some people foundling me before! For this reason, I have started showing off "whats underneath" at the Greenfield Village's fashion show.
There is no doubt of my thoughts on wearing historical underpinnings. And to give you a little TMI, I wear absolutely NO modern clothing while I wear my historical clothing. If you dont understand, you soon will.
I know how becoming a reenactor you just want to jump right in! You want a fancy ball gown and all this fun lacey stuff! And no doubt wearing all of that IS fun! But to save YOU time and money, first we must train our eyes and be realistic.
We must study and recognize the different shapes each era creates. I have laid out three distinguished fashion plates within 50 years a part. Like todays world, you would not want to be wearing your grandmothers clothing (unless your reenacting 1940-1950's lol).
Take time to exercise your mind and see the slowly, but surely, transition from one era to the next. I find it fascinating that one small change from year to year eventually makes a large change between 15+ years later!