messing with the plants hormone ratios can make the plant grow weird. You can do that, but you might see weird things happen....and it may not be beneficial. they generally don't need a massive amount of new stuff when flowering so they don't 'root' much more. You know how they fade and eat the nutrients in the leaves? it's their program, they do grow more roots but it's not a priority.
Some of the growers feel that the plant will slow down and almost stop new root growth and development once their plants start flowering. When in the flowering stage this starts seems to be about a week after the pistils start to show and for sure about 2 to 3 weeks after the first pistils.
Keeping what @Mudballs mentioned about plant hormones in mind I am thinking that it would be better to add the rooting hormones about 2 or 3 weeks before the lights are switched over to some sort of 12 on and 12 off schedule.