The first question is really up to you, and depends on your preference. If it was me I'd use the screen to help aid with horizontal growth and training the plants. As for the last question what strains are you running, is your light is cooled? typically in scrog growing you want your plants to grow at a pretty even rate, so that you can develop an even canopy and not have one plant (or cola) sucking all the light from above the screen while your otheres are struggling to survive.