You could just say you've never signed off on one, and that when you eventually do, you'll read all of the terms because not knowing all of them could have a bad effect in the future. For example, you could say like "I have never signed off on a 'Terms of Agreement.' But, if I encounter one in the future, I will be sure to read it all the way through. I would do this because if I didn't, I could miss an important term and accidentally violate it. Then, I would get in trouble for doing something that I didn't even know I wasn't supposed to do."