You mean have the light on for 8 hours rather then 12? I wouldn't say it's a good or bad thing IMO. I know some folks have a 36-48 dark period right before they chop, to basically make the plant believe it needs to pump out as much energy to make juicy buds.
But 8 hours of light a week before harvest probably couldn't do much damage because you're practically done, and at the same time I'm not sure of how it would benefit exactly.