When it comes to search engine optimization, I generally subscribe to the Just In Case theory. If Google *might* penalize an action, do not take the action - Just In Case. If Google *might* reward an action, take the action - Just In Case. All seo work is based on educated guessing, with hopefully, some sort of logical research to back up the guesses. On any given day on the seo forums, you will find an abundance of rumors, theories, and even conspiracy theories. Many of these, however, are logical, and some of them are backed up by some serious research. These are the ones that I would put into the Just In Case category.
For example, the rumor that pages called links.html (or any links.tld) does not pass PR, or is discounted by Google, is one that many people have researched. After several months of arguments amongst the seos, it appears as though Google *might* discount links on a links.tld page, at least some of the time. Perhaps they don't. If you already have a page called links.tld, then it may not be worth it to rename it. However, if you are starting a new links page, you might as well call it anythingelse.tld, JUST IN CASE. Why flirt with a potential problem?
Another example is the theory that a page that validates will be rewarded. I can certainly agree that non-validating pages are not penalized. If they were, there would be millions (billions?) of pages that would not show up in the SERPs - and they do show up. So W3C validation is certainly not a requirement. However, is it possible that validated pages get a small reward? Probably not much, if it does at all, but every little point helps, right? In
addition, a page that doesn't validate *might* have one fatal flaw that prevents Google from spidering the page completely. So, if at all possible, validate your pages, JUST IN CASE. You can validate your markup at http://validator.w3.org.
There are many scenarios that could fall into the Just In Case theory. The point is, think logically about the action you are about to take when optimizing your page. Does it make sense to take the action? Would doing so *possibly* cause a penalty or discount? Would doing so *possibly* cause a reward? If the logical answer is possibly or probably, then place it in the Just In Case category, and react accordingly.