My big test-prep calculator guidance amounted to "STOP PICKING UP THE CALCULATOR!! Just use it on the desk for Pete's sake!"
DS had never used a calculator before his first Explore, so we did a tiny bit of math each day with one so he'd know where all the buttons were. And so he could remember to just leave it on the desk please. I don't think he would have finished any of the sections with or without a calculator (it wasn't just math that he didn't finish) but he was very young for the test, and even without finishing he did well. He did much better the second year when it wasn't all new. That year I think he did finish most sections.
My favorite calculator for this is the TI-30X IIS. They're cheap, and they have two lines of display so you can see what you entered with the results. Also, when you do switch to a graphing calculator, all the buttons are in the same place as they were on the TI-30. For the Explore there wouldn't be any particular benefit to a graphing calculator, but last year that's what he used, because that's what he had been using all year. The most important thing is that they not be trying to figure out the calculator while they're working on the test.
