Compression is but one of several tool used in the mastering process (or none at all depending on the tune!). In any event I would wait and compress along with whatever else you are doing in your mastering process unless your are using an anlog compressor for the sound and the rest of your matering is done digitially. you would have no other choice.
As for the highest peak comment I would say that also depends on what you plan on doing in your mastering process. If your highest peaks to dat reach zero you have no headroom to work with. If you add any eq, for instance, then you will get digital distortion. I would leave a few db headroom going into the dat if you plan on further altering it later. you last steps should be placing it at the right level you want and then dithering (if necessary).
Kevin S. Mucha
Coerce Recording Services