I agree with Cheryl on this one. For better or for worse, figure studies are a special case. While I think the photographer needs to protect him or herself by not giving away negatives, I can imagine destroying negatives in rare cases.
If you don't want trouble, then the solution is to hire professional models who understand the legal issues and pay accordingly. If one relies on friends and acquaintances or inexperienced models willing to trade time for portfolio prints, then I think one has to acknowledge a certain degree of risk and the possibility of regrets down the line.