The textbook answer is you should do what the Approved Data tells you to do, otherwise if anything goes wrong your ass is most surely grass.
However there are applications where it is quite normal to re-use packings, subject to inspection for cuts, flats, chips and rolling. These are not all 10-cent items by any means - some larger ones can be well over $100 apiece, and if they are being exposed on a regular basis it can make a big difference to your maintenance costs to re-use serviceable parts. And it's perfectly safe as long as it's done in a controlled manner with subsequent leak checks.
The key is in how you do it. For seals that get exposed once in a blue moon you're probably better off being safe and changing them. But if you're in an environment where you have control of your processes (and I'm thinking processes derived from CMMs and adapted to local working practices) you can then write into YOUR OWN approved data which packings can be re-used, along with the circumstances and any associated leak checks post assembly.
If you read the introduction section of just about every CMM in the world you will see a statement that the manual contains RECOMMENDED procedures and the manufacturer permits deviation from these to suit local working practices if an equivalent or better standard of safety can be achieved. The art is in justifying your own procedures in line with this statement, and everyone does this to a greater of lesser degree whether it's use of different paint, cleaning systems, tooling, etc.