My humble opinion, the best way to beef up the rear frame rails are to replace them with solid 2x3 units.
Lots out there to choose from, and one of the more established companies is Autoweld (scroll down):
http://autoweldchassis.com/spec.ivnu
In the absence of that, if you're mini-tubbing you can put 1/8" steel reinforcements or box the internal structure of the rails. Then there's the Mark Stielow way which can be found in his recent build thread post #70.
I'm not sure what the answer is (i.e. do it or don't), but if you're in the media blast stage and want to reinforce them now would be the time.