If it's just sitting there on a table waiting to have the alarm go off then leaving hold on isn't saving any battery

If it's somewhere that the buttons could get bumped by accident then leaving hold on will save a little power (the amount required to spin up the drive and load the original firmware, then keep the LCD running until the OF decides to go to sleep again).
I'm (slowly) working on a series of changes that will allow us to stop using the hold switch for this purpose to prevent, amongst other things, the exact problem you are having. No ETA right now, though. (it's more complicated than you might assume).