For the
trim lines, we usually only take about 1/16th of an inch from the
outside edges. It's not a ton, so usually I wouldn't even worry about
it. For the center, nothing is lost in the middle fold, so again
nothing special to set up there.
All of that being said, two things to keep an eye on:
1. Heads or mission critical items in the center of the spread - While you don't lose anything in the center, it can be distracting to have a face right in the fold
2. Images with little to no head room against
the edges - If you cannot afford to lose any part of an image, make sure
you give it a little extra space from the edges, just to be on the safe
side.