Once you have changed the permissions to 777, I doubt you will need to them again for the next upgrade if you upgrade via the Moodle user inteface.
If you upgrade some other way, like downloading a zip file and unzipping it on your server, then you may need to change the permissions.
Julie, Elek: now you know that 777 is the required permissions for 3rd-party plugins to run on your Moodle site on your server, then it would be wise check the permissions after the next time you upgrade, to discover whether or not you need to set permissions every time.
Note that not all servers need these permission settings. In fact, in my experience most servers do not. It all depends how your server is set up, and how you upgrade plugins.