As a developer working on SharePoint customizations within Microsoft, I can attest to the end-user benefit of a clear and concise customization policy for any MOSS deployment.
Sean Livingston has made available a document which was the basis for Microsoft's own SharePoint customization policy, and that you can use as a starting point for creating your own policies.
Save yourself some time (and impress your manager!) … Grab Sean's whitepaper and put together your own customization policy today.
Abstract from Official SharePoint Product Group Blog:
The SharePoint Products and Technologies 2007 Customization Policy white paper is a starting point for companies to develop their own policies for implementing and managing customizations in a SharePoint based environment. A version of this document is currently being used as a key component of the hosting policy for SharePoint customizations within Microsoft's internal SharePoint hosting team, which manages what may be the world's largest SharePoint environment. This document was designed to act as a bridge between system administrators and developers so that both sides could evaluate SharePoint customizations with an emphasis on administration concerns such as supportability, performance, and deployment issues. This document will be added to over time as new information and best practices become available.