Sean's SharePoint Ditty

It is easiest to manage permissions at only the site level, whenever possible. This means you should create your site hierarchy in a way that allows you to assign permissions to sites that are appropriate to all securable objects within the site, such as lists, libraries, folders within lists or libraries, documents and items.

Although you can assign unique permissions on any securable object in the site hierarchy, to do so is more cumbersome than inheriting permissions.

As much as possible, arrange sites, subsites, lists and libraries so that they can inherit most permissions.  Put sensitive data into separate subsites, lists, libraries, and so on.