Also check out the tutorial on Super Cropping, as another tool for that. Just started doing that with my girls, since they are reaching for the plastic!
Whatever you do, don't let them grow up against your plastic, or even get super close to it. I have seen other greenhouse growers throw away Lot's of bud because they got against the plastic and got frost bite and bud rot. If the plants are going to get super big, think about just cleaning out all that little stuff in the bottom middle of your plant, and keeping the bud sites toward the outside and top of the plants, where they can get plenty of light, and a good breeze to help prevent mold problems.