This is a sort definition from Google:
It is best to use EXPLAIN PLAN to determine an access plan, and then later prove that it is the optimal plan through testing. When evaluating a plan, examine the statement's actual resource consumption. Use Oracle Trace or the SQL trace facility and TKPROF to examine individual SQL statement performance.
You must perform some tests when the server has many or max users and with the testing you are going to conclude for the value of the plan, The env is rarely static, you have to follow the evolution of the DB.