I've managed to halve the voltage (1.2 to 0.6) of my Radeon HD 5770 (with some minor underclocking).
Can this damage my GPU in any way? Is it safe to go even lower?
And no, I'm not joking.
If you have under clocked it with standard tools, TRIXX, Afterburner, etc then you will not damage it. If you have done it using a modified BIOS then providing you haven't got it wrong it should be okay, you can get a setting for something else on the board wrong that screws it up.