I've got a HMMWV airlift bumper for mine. It's 3/16", and probably weighs #150+. I think 1/4" would be overkill.
But then I like overkill...and I just remembered that the stock bumper is single-side 'c'-channel, not box tube. 1/4" might be the way to go, if you're going to keep it stock/open.