In your c:\documents and settings\[YourUsername]\Application Data\Subversion there should be a file named config. Open it in a text editor. There should be a section named [auto-props]. It’s probably toward the bottom. In that section, add a line like this:
*.cs = svn:eol-style=native
where cs is your file extension. You’ll want to do it for you file type (cpp, html, pas, h, whatever). Save the file. It should no longer hassle you about eol-style when adding files.
Don’t do it for binary file types. Also, understand why you shouldn’t.
This assumes a default installation.