Hi Sarah,
It depends on how many plates the job uses and what stock your designs are on. If you can fit your work onto one plate and they are on the same stock then generally it doesn't matter
We normally ask for the artwork separately and plate it up as we see fit in terms of whether we run it with another job etc etc. ( I work a bit in a printers)
Dont know whether this helps you, i think ive confused myself tbh!