We test all of our heavy hitters: SO Entry, WO Entry, PO Entry, G/L Posting, MRP, Cost Rollup, etc. Plus we test our reports, especially ones we have to run up on the server and then send the output to Optio for processing.
I believe that any document output package (Optio, FormScape, etc.) may have sensitivity to Service Packs, since Service Packs can change how .pdf files get generated and/or formatted.
Basically, when in doubt, test, test, test!