i think this one is personal preferance but i will aid you in making that decision for yourself, flushing with fresh water is always a good idea to remove excess salts from your res, however flushing your already fully vegged plants ready for flower with pure water for 7 days will be starving your girls, i definatley agree with routine flushing of your system, but as far as removing all the veg nutes before switching to flower nutes makes no difference, your lights are what makes the plants flower, all your nutes do is cater to the specific diet they need in that particular stage of life