Since presses print in large signatures it is important to swim downstream and work with the press equipment when printing books, magazines, catalogs or other periodicals. When finalizing your files you must keep in mind not just how many pages you end up with but how many signatures for the best pricing on the press. Odd signatures are more expensive then complete ones. Commercial presses today print in 16, double 16’s (32), 48 and 64 page signatures (the last two for smaller page sizing of books).
Web presses print on rolls and not sheets, thereby making them more anal and less able to vary on what is considered a press friendly run. Sometimes, as in magazine printing, where you are selling advertising, it can be more cost effective, thus giving you the lower pricing to add pages and sometimes 8 or 12 more pages can actually save you money due to the fact that you would be utilizing the best page count for a given press. As a result your per page price decreases and thus your advertising profit when printing magazines increases.
The bottom line is, always swim downstream when you are on a commercial printing press. In the end you save money by doing so.