Check out SuiteAnswers 50298. It provides a test plan and template that you can tailor for your company, designed to complete in the release preview. We used this template and for each workflow we assigned different NetSuite roles for testing. Each test case is a different task they complete in NS. We basically delegated different users in the company that have different roles and tasks. We then had them complete testing their basic tasks in NetSuite, and then log the results/system behavior for each task. This way we could catch any major issues that could affect the day-to-day duties from the new release.