Unsetting vars when not needed?

Hi all, I was working now in a new site developed with PHP 5.0.2 and a wonder came to me: Is usefull and recommended to unset a class instance when not needed? Specially when the class was instantiated inside a function and the function ends. In this case, the system should automatically destroy the instance (as it is a local var inside a function). If we manually unset the instance at the end of the function, does this affect the PHP performance? I mean, will PHP have to do an extra job unsetting the var? Also, the same question arises with database results. Is it worth to call mysql_free_result at the end of a function that uses it? Must take into consideration that there are lots of functions that use lots of MySQL results. Thanks for any comment Jordi.