I'm pretty sure the ice water flush is to try to get the buds to purple up a bit, which does have some scientific basis. Light is known to degrade THC, so I always try to harvest right after the dark period. The dark period is when the flowering hormones build up, which is the theory behind an extra 24 to 48 hour dark period. Turning the lights off for a day before you harvest is not going to hamper your crop, unless it is already on the edge of problems. It may or may not increase frost, but it will definitely save you power for every day you leave them in the dark.