Moss custom master page Best Practices

Do not change the default.master
Even the OOTB master files are installed as feature - PublishingLayoute feture.
Use Sharepoint Designer to change the default.master, .css files and images to your requirement. Do not save in Desinger. Open any editor, and copy from SPD and save.
Use the copy of PublishingLayout feature deleting the unwanted files and copy your master page, .css and images.

Create a .wsp solution file( Use tools - stsdev or wspbuilder )
Although we can copy the new master feature manually and activate, deploying as a solution will releive you from the pain of copying into each WFE in the farm.