Yes To Underlayment......
When warm, moist air comes into contact with the cool underside of the metal roof, condensation will form and drip onto the wood sheathing below. In the Northeastern USA / Eastern Canada, where I install, temperature fluctuations from extremely cold to warm are the norm and metal roofs can sweat to a significant degree. In Texas, it will be less but your money will be well spent in protecting the wood sheathing. I recommend either a 30 # felt (tar paper) or a synthetic underlayment such as Grace Tri-Flex 30 or IKO Roof Top Guard.