I've done a little research into this very topic. From what I've found it was done, but not required. From what I've read there really weren't any carved in stone rules about helmet markings. It varied from Division to division. Like you said, the Tech ranks did not have authority to issue commands. But some of them still found themselves in positions where they had to. I've also read that the Tech ranks caused a lot of confusion and frustration. Prior to their creation ( I think about 1942) there existed a group of pay rates called First and... For example First and 5Th, was a PFC who received Cpl pay for the specialist job he did such as mechanic or Cook or Baker. He became a T5 when the new ranks were introduced. Where some real confusion came in was in instances where let's say a Cpl receives a "promotion" to T4, more pay, but suddenly less authority. From what I've read this happened and caused a lot of consternation.
Hope that helps a little.
Tim