Well, that depends.
Are you running one or more queries (SELECT, SHOW, EXPLAIN, DESCRIBE) that are expected (or can possibly) return large result sets?
This function will ONLY have any noticable affect if you answered yes. It should also be noted that if you only have one or two queries return large sets, you probably don't need to use this function.
If you read the manual entry for [man]mysql_free_result/man, you'll see that PHP automatically clears the memory when your script is finished executing, so there's really no need to worry about it unless you have multiple queries in your script and would like the greatest amount of memory available before a large result set is returned.
