Always approach the booth with a smile. Been to US many times. I always say 'Hi, nice to be back' Never had a problem. Even found I hadn't filled the 'Green Card' in correctly once, and didn't get sent to the back of the queue, which apparently used to happen. TSA lady corrected it for me with a smile, and the usual 'Have a nice day' which [for once] sounded sincere. Always nice to visit the States, and nice to come away.
