changing behaviour of notify per post

Hi,
a little request:
When a user register to the site, its default setting is to not receive email when someone replies tohis post.
My opinion is that this feature should be active by default, not disabled. Do you think it could be a good idea let an user decide this during the registration form?
Or, could you add a parameter in the administration settings to decide if this option can be on or off per default?
thanks in advance.