Sure, you can do anything you like actually. Post up some pics would be helpful, it's always a judgement call IMO.
This is a highly charged subject, ask a dozen people, and you'll probably get half dozen different answers on this one.
I'm kinda against doing defol actually, but that said, I do often start to do so in the later flowering stages, as the canopy really thickens up, so that the bottoms can get some light. Usually depends on how the grow progresses for me, and the strain in question, some simply get much thicker than others.
It also helps with ripening uniformity toward the end, IMO. Getting those lower spots ripened on par with the tops, either that, or simply harvest in layers.