I always run a test site before I do production. Usually upgrade test from the last version before I put them on production. Always something slips through the cracks.
I have a list of things I check if the upgrade process completes ok including:
Can I register a new user
Do I get emails
Can I edit pages
Do any modules act different that are most critical.
Can I edit portal sites
Can I create a portal site
Can I edit users
Can I Create a user
pretty much everything in the persona bar.
then just general browsing user administration activity.
Submit bug reports on github as best I can afterwards.
It would be nice if this was all broken down into groups and groups within this group could look after testing the different sets of DNN functionality. It sure would give me more confidence putting something new into production from DNN CE.
Count me in :)