OK, so this thread has a long discussion about what is meant by "powered off". If you mean you press down on the Sleep/Wake button until the Slide to Power Off comes up on the Home screen, and then you slide it, then in that state the alarm didn't go off in 2013, and it still doesn't.
The alarm will go off when the device is in Sleep Mode (i.e., you press the Sleep/Wake button to shut off the screen.
The device cannot be overcharged, so you don't need to set any alarm for that reason. I have iPhones that stay on a charger for several days. My iPad pretty much is always hooked up to the charger. They absolutely cannot be overcharged.
And powering the device off to charge it is totally unnecessary. Not sure where you heard it was the best way to charge a mobile, but that is simply not accurate.
Best of luck,
GB