This is an interesting question and one that is rooted in the history of cinema. Before the days of readily-available digital workstations and non-linear editing, film was cut on just that: Film. This magnetic medium had its limitations, one of which being how much film could actually fit on a reel. There were "A" reels (around 1000') and "AB" reels (around 2000'), and the picture editor would cut accordingly. For example, if the action scene in Reel 3 was going to go beyond 2400', the film editor would have to find a place to cut before the end of said scene since it physically wouldn't fit on the reel. Other factors figured in as well, such as music cues that couldn't transition smoothly from one reel to the next.
Well, now we're firmly planted in the world of digital non-linear editing, not just in sound but also in picture - and, very soon, also in the realm of theatre delivery, ie. "digital cinema", where the theatres no longer receive reels of film but rather computer files downloaded from secure FTP sites containing the entire movie, with high-definition picture and surround soundtracks.
Where does this leave the lowly A and AB reels? That is the subject of some debate. Since file sizes and download speeds are not much of an issue anymore, there may not be much need of reel breaks in film anymore. I've worked on entire projects that were delivered as one file, rather than "R1", "R2", etc. etc. The whole notion of reels in film seems to me a holdback to an earlier time, and not particularly necessary with today's technology.
In conclusion, I would encourage you to consider alternatives to the tried-and-true traditions. Maybe work with the entire film in one file, with no reel breaks? You may enjoy the continuity and flow. Ask the picture editor his/her opinion on the matter, and see where it goes.
PS. All of you film veterans out there, please chime in and correct me where I may be wrong. I entered the world of feature film editorial in 1997, an pivotal time when non-linear workstations were coming of age and mag film was on its way out. As a result, I have pretty limited experience working with mag and film, and some of my terminology may be kinda off.