I keep everything that I do in a subversion repository. Even though subversion can diff binary files like Word documents or OpenOffice.org ZIP file enclosures for documents, I still like as many of my design artifacts to be plain text as possible. Now, I do keep lots of binary files in repositories, especially when working on book projects, but I do have a strong preference for text files.
I also like my design artifacts to look good, even if I am the only person who sees them. Two highly recommended tools are AbiWord and OmniGraffle because their default file formats are plain XML text files.
I admit that disk space and network bandwidth are close to free now but I still like to keep a project directory small. By using design tools that have small file footprints, most projects (source files, build scripts, tests, and design artifacts) are small and tidy.