It's a good idea to reinforce. One thing you can do to prevent warpage, whether or not you reinforce, is applying resin to only a portion of the helmet, letting that cure, then a bit more to another portion, letting that cure; continue that until the entire helmet has a coat of resin, then apply a second coat over the entire helmet. The main reason peps warp during resin is because the resin soaks into the paper, disrupting the structural integrity while the resin is still wet. By limiting the amout of wet paper at any given time, you'll decrease the risk of warpage.
Just FYI, these types of questions should be directed to the proper Help! or FAQ threads stickied in the NOOB forum.