When you go about ‘cleaning’ up irrelevant out of the box components of your CRM system whatever you do, don’t delete the System Dashboards. This will likely create unpredictable issues for you downstream. For example later on if you try to import an ISV managed solution that references a dashboard that it expects and it isn’t there the import will fail.
The best practice is to use security roles to hide them from your users.
In the example below we changed the default status from “Display to everyone” to “Display only to those selected security roles.” We then unchecked all the security roles. Therefore, they wouldn’t appear in the UI to any user, including the CRM admin. If the Admin needed to see it they would go to the customizations area.