Apparently it is safer for you to release the brake just before impact in an accident like that b/c leaving your leg on the brake means your leg will be absorbing all of the momentum shift that occurs whereas leaving it off the brake it won't absorb any impact (unless there is crush damage in the cock-pit)... I thought that was interesting information!
It is also a very good idea to let go of the steering wheel just before impact. Often the steering gets yanked around because one or more wheels get hit, and you might break your thumbs.