robots.txt files aren't particularly important, unless you get annoyed with the 404 entries in your logs, or you're trying to keep a particular search engine out of part of your site.
For example, if you built a set of pages specifically for Inktomi on
http://www.yoursite.com/inktomi/ and another set for Altavista on
http://www.yoursite.com/altavista/ you could do the following:
User-agent: *
Disallow:
User-agent: Slurp
Disallow: /altavista/
User-agent: Scooter
Disallow: /inktomi/
This will keep Scooter out of the inktomi files and Slurp out of the Altavista files.
For more information check out the following website:
http://www.robotstxt.org/wc/robots.html