Trimming is a mechanical process and like most such process is not accurate to the nearest mm and is not necessarily repeatable in subsequent orders of the same book. So I tend to treat the scissor lines as the most likely place that the pages will be trimmed, but I never put any critical content into the pink areas.
For full bleed pages I put the photo right up to the edge of the page knowing that part of the photo will be trimmed. Putting the edge of the photo up to the edge of the scissor lines can lead to a page with a thin white strip around the edge if the trim is a little off - this looks awful. I also make sure that no critical part of the photo, such as the top of someone's head, is in the pink area.