If you include an eBook ISBN in Atticus, that information will be encoded into the data for the eBook, so this isn't something that you'll see unless you're looking at the books coding. This information is not automatically added anywhere in your PDF (print) files, so it's not necessary to add this information for each of your print book formats to the Book Details page unless you want it present for your own informational purposes.
Since the most important aspect of ISBNs is including them when you're entering your information on your dashboard at the retailer level, I highly recommend checking out the articles available on Kindlepreneur.com. They have a lot of reference information on publishing, including acquiring and using ISBNs.