We have recenting been working with MEAA to build new functionality in to their CiviCRM in order to build their capabilities.
One of the requirements that arose was to be able to set some Activity Types to send notifications to the Assignee without triggering them for all Activity Types.
This feature was accepted by core and is now available in 4.7.30 release.
Now when you visit Display Preferences at civicrm/admin/setting/preferences/display?reset=1 (red box) you will see a select option to specify any Activity Types (green box) that should be excluded from notifications going to Assignees.