There is no single best practice for determining which test cases to automate, as it depends on many factors related to the environment, potential impacts, and costs versus benefits of automation. Some good candidates for automation include repetitive tests, stable features and modules, smoke tests, tasks with complex calculations, tests requiring regular environment setup, and tests that are difficult for humans to perform but easy for computers. The decision should balance the potential return on investment from automating tests against the effort required to automate.