Florakleen is only used to flush nutrients out of the plant. So there is virtually no reason to use it for more than the final week.

Now on the bottle, it says you CAN use a little before you change your res, but I think the only time you might is if you have some serious nutrient overferting or something very unusual.

If you used it at every res change, you'd be basically in a cycle of sucking everything out of the plant, then replacing it from square 1. I'm not sure how the plant would like that. If you had tons of salt buildup or some signs like that during grow, then yeah someone might recommend to flush it as good as you could to save it.