You dont need to back an old bow 'just because'. What I have done with success is to put them on the tillering tree, unstrung, and get the limbs bending with the long string, exercising it 40-50 times at each inch increment, and only go to the next inch if tiller looks good. Once it's gone far enough that it can be braced, put the proper length string on it. Continue exercising limbs 40-50 times at each increment, giving it thorough inspections every so often, and continue until full draw. If it holds up and all seems well, go shoot it.