You can flower at any time; It depends on all the other factors involved with your grow as to when your plants are big enough to flower. If you don't plan to train them at all I'd personally wait because they are not close to big enough to maximize their potential. That said it is totally impossible to answer whether or not you should flower or the yield they will produce without knowing all the details of you project. How about strain, room size and height, actual lighting configuration, how much you'd like to yield, and how soon you'd like to finish?