One of my favourite quotes from Google is:
A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
I like that quote because it makes a lot of sense and is easy to understand. Any changes we recommend for SEO also clearly have benefits for users too. It may just be the case that the search engine involvement completes the business case required for making the change.
Google's Sitemap XML program broke the (rule of) thumb. It was just for search engines.
Adam Lasnik has posted a very good Webmaster Central blog post on duplicate content. In particular, I'm pleased to see a tip to be consistent with internal linking. I don't understand why webmasters insist on linking to /index.html when / would work. If you insist on linking to index.html it means is that your upgrade to PHP will involve awkward URL rewriting rules.
Adam also writes;
Rather than letting our algorithms determine the "best" version of a document, you may wish to help guide us to your preferred version. For instance, if you don't want us to index the printer versions of your site's articles, disallow those directories or make use of regular expressions in your robots.txt file.
Hmm. Tough one to squeeze into "I'm comfortable describing this as a non-search engine specific strategy to a competitor" box. Google's broken their rule of thumb too. Clearly, though, this is talking about excluding pages from the index which is a far cry from trying to manipulate the algorithm into ranking you higher. Sometimes the best rules of thumbs are those you can define by the exceptions and perhaps this is one.