Micheal's site "Says" that the hygrometers should be calibrated out of the box, but it also says that shipping and such can mess up the readings on a hygrometer.
Sooo.... You should calibrate your hygrometer. I ordered one of the calibration packs with my hygrometer, it was only a dollar. I would trust the pack over the salt test anyday.
I Never Could Get The "Salt Test" To Work Right. I Always Use The Boveda Calibration Packs!