This is an excellent guide to all possible customizations in SharePoint 2007 including real world examples and support details. Lawrence describes it as a starting point for companies to develop their own policies.
Abstract: This policy document outlines the items and processes for managing customization within an enterprise Microsoft® SharePoint® Products and Technologies hosting environment. This document begins by describing the service management concepts relating to the hosted services. It then presents information on commonly requested customizations and a few customizations that are not commonly requested, but which present significant risk to the hosted environment. Lastly it details some service policies that might affect customization planning or implementation.