My father used to tell me, "whatever you do, be consistent." I don't specifically remember the context in which he said this, but being consistent is increasingly important when it comes to organic search engine rankings. Let's take a look at why you might be shooting yourself in the foot if you aren't consistent in your SEO efforts.
Keywords are Key
Search engines have been known to give some credit when you have your keywords in your URL, especially if that page has the keywords in the URL and the page's content includes those keywords. Naming your pages consistently has its benefits.
Keepin' Your Internal Linking "Real"
Now let's look at the home page of your site. I would consider your home page to be located at http://www.example.com/, not at http://www.example.com/index.html or http://www.example.com/default.aspx. Even though the index.html or the default.aspx file is your site's home page, the real home page is http://www.example.com because that's the page everyone tends to link to when they link to your Web site.
So, why should your internal navigation link to the index.html file instead? Your Web site's "home" link in your navigation should link to your real home page (i.e., http://www.example.com/). If you aren't being consistent when you link internally on your site, something you have complete control over, then you're losing out on better search engine rankings.
It's quite possible that your internal linking inconsistency isn't passing "link credit" around your site properly. The home page is the most important page on your site. Why pass some of your "link credit" to one URL (http://www.example.com/index.html) while other "link credit" is being passed to another URL (http://www.example.com/)? It's important to fix the internal links to your home page, and consistently link to what I call your "real" home page.
Home Pages and Redirects
Different Web servers use different files to serve up the home page of your site at http://www.example.com/. A Unix Web server may use index.html as its home page file, or it might use index.htm. A Windows server might use default.asp, default.aspx, default.htm, or default.html.
In any case, it's a simple change in both Windows and Unix Web servers to specify which file is served up when a visitor requests your home page at http://www.example.com/. On a Unix server, the change is typically made in the configuration file. On a Windows server, it can be as easy as highlighting the file in the administrative interface (similar to Windows Explorer), right-clicking on the file, and selecting that file as the home page of the Web site.
To this day, I come across many major Web sites that aren't consistent; they redirect their real home page to another "home page" on their site.
For example, the local NBC affiliate in Dallas, Texas uses an Apache (Unix) Web server and has set up a 301 permanent redirect from their home page (http://www.nbc5i.com/) to http://www.nbc5i.com/index.html. Changing this setting on the Web server to specify that index.html is the home page of the site and removing the redirect will help their Web site's search engine rankings. Furthermore, their site's logo (on the top left of the site) links to http://www.nbc5i.com. Yet clicking that link redirects to http://www.nbc5i.com/index.html.
When it comes to your domain name, it pays to be consistent. You have control over the internal links on your site, and you have control over whether you're using www.example.com or example.com. Again, being consistent by choosing one (either www or non-www) will help your site's search engine rankings. Allowing the search engines to index both the www version and the non-www version of your Web site will potentially cause duplicate content issues and other search engine ranking problems.
Taking a look at your Web site's consistency -- not only in your internal linking and what you're linking to internally, but the consistency in the use of your domain name -- can make a big difference in your site's organic search engine rankings. Make sure you're not shooting yourself in the foot by being inconsistent.