For a variety of reasons, that's not always possible.
For example, the shape of the document relative to the press sheet may not make full use of the available space as in this poster:
Or the document may include odd-shaped die cut elements as in this carton package example:
Those "off-cut" blank areas needn't just go into the recycling bin. They can be put to good use by adding another document that simply tags along with the proper job. Perhaps a promotional piece:
Or to print test elements to gain knowledge for future projects:
If the print buyer has a good relationship with the printshop then, typically they will be charged very little, if anything, to add these kinds of files to a print job. Here are a few guidelines to keep in mind:
1) Always ask for an imposition layout form when a job is being quoted. That will indicate whether there is any available off-cut space.
2) Get permission from the end print customer to run a tag-along file.
3) Make sure that the press operator understands that the main client job takes priority on press.