Hi Sillymethree,
From this site:
http://www.bachmans.com/tipsheets/Perennials/HardyHibiscus.c fm
"Pruning/Pinching: If you want to encourage better branching, pinch them lightly early in the growing season. Flower buds form on the ends of branches. Pinching too late may delay blooms. Otherwise, they shouldn feett need pruning unless they grow too large for their space. Once the new shoots have emerged the next season, you can prune out any old stems."