Most folks will tell you that you should never modify the default
masterpage and the core.css file.

Instead, you should make a COPY Of these, and then use that copy of
the files. The reason being is that there is no guarantee that a
future Service Pack might over-right the default masterpage and css
file thereby destroying the work you did.

Copy them and then just append to them.

I should note that the OWS files are now called CORE and there is a fifth
solution that WSS v3 now allows in the form of an HTTP Module.