Define what data points you need to report on before creating custom fields. What information needs to be queried to return a list of issues vs. what information just needs to be visible in a single issue? Example: You collect a serial number for a piece of equipment. What will you do with that information? Will you produce a report of all serial numbers in all issues? If so, this is easiest with a custom field. No report needed? That piece of information can be included in a standard field instead. Read Atlassian’s excellent article on “Field Greatness” here: jirastrategy.com/link/field-greatness.