I agree with the responses you have thus far, however, I UT every full pen joint before it leaves our shop, regardless of contractural language between the customer and our company, just to cover our behinds. I want to be certain that there will not be any surprises in the field, should the customer decide to UT a few joints in the field. We have a great bunch of welders and I have confidence in their ability, but anyone can have a bad day, so I check them all. Besides couplant is fairly cheap :)
John Wright