One wire is live in that it moves between —x and +x volts in a sine wave. The neutral is often connected to earth/ground somewhere upstream and should be 0v. So if you just pop that power into a transformer and then regulate and only use the DC then it's completely irrelevant which way you plug it in.
However there's plenty of cheaper ways to get a lower voltage, or to power a device from mains power that when done badly can reference what is thought to be neutral (and this ground) but could easily become live and make something metallic become live either without fault, or with fault to a minor component.