It's inevitable and not cheating IMO. Sure, I try and not do it, but I don't consider it cheating (yes, even if you're camping, sniping or hiding or what have you). Fighting games let you see your opponent the whole time, you see what they do and anticipate things by their actions. Sure, it's a little different in a shooter, but that's what you sign up for being split screen. That's why I love LANing. One team goes on one TV, one on the other. Then screen peeking isn't to know where your opponents are, but where your allies are.