Etiquette of letting a friend cut ahead of you in line when others are behind you...?

I am planning to go to a midnight showing of Harry Potter and the Deathly Hallows Part 2 this July. I want to get some really good seats, so I want to show up at the theater nice and early. This being the final Potter film and all, I am anticipating the lines getting long maybe several hours before showtime. I'm thinking if I show up around 6 or 7 pm, I should get good seats. Problem is, some of the people who are planning to come along with me may not be up for standing in line for five or six hours. Is it improper to allow these late-comers to cut in front of me when there are people behind me? They are cutting "me," not someone else, right? Or are they still cutting everyone behind me who has been waiting?