This is an excellent question. As someone who manages 800,000 images and growing,I would never get rid of negatives unless I have completed a full assessment of the collection by asking questions such as:
Do these items fit our institutional mission?
Do these items have significant preservation concerns?
Do these items require special treatment/storage beyond what we already provide?
Do these items have archival value?
Do these items have copyright issues?
Do these items have ownership issues?
Are there any restrictions?
What is the size of the collection? Is there a chance of an addition to the collection?
How are the items currently arranged? Will we keep this arrangement?
I also think of them in terms of:
Negatives as the master images. They are the permanent record.
Just because you have a print of the negative doesn’t mean you have all the information you need. A print is an interpretation of the negative and no two photographers will print the same negative the same way. The print may also be deficient in its details compared to the negative.
A digital image in this case is merely a surrogate, much like the print.
The Library of Congress has an amazing bibliography, http://www.loc.gov/rr/print/resource/vmbib.html
You may also find this helpful, http://www.loc.gov/rr/print/cataloging.html
I also suggest
Chapter 4 in Mary Lynn Ritzenthaler, Diane L Vogt-O’Connor, with Helena Zinkham, Brett Carnell, Kit A. Peters. Photographs: Archival Care and Management. Chicago: Society of American Archivists, 2006.