A few years ago, it was a no-brainer. If you had a Web site that had any number of dynamic variables – such as ?, =, or & – in your URLs, you'd want to rewrite those URLs so the search engines could better crawl and index your site's pages.
Now, with the advances that search engines have made, along with issues that some search engines have with redirects, the answer isn't so clear.
The Case for URL Rewriting
If you're redesigning your Web site and the URLs must change, that's the perfect opportunity to implement SEO-friendly URL structure. Consider adding keywords to the URL string (www.sitename.com/keywordA-KeywordB/) and rid yourself of some dynamic parameters that exist (explained in more detail, below).
If you aren't redesigning your Web site, but your URLs look something like "http://www.sitename.com/site/olspage.jsp?id=1828934&skuId=8745423&type=product," consider rewriting your URLs. There are about six dynamic parameters within that URL string, which could cause problems for both search engines and users.
In some cases, a site's URL structure can have a dramatic impact on rankings. One popular Web site I know of, which has every chance in the world to rank well for their product/keyword, doesn't even show up in the first three pages of Google search results because of the site's URL structure. By all rights, the site should be on page one, and it isn't a competitive keyword.
The Case Against URL Rewriting
If the pages of your Web site are already well indexed in the search engines, you have good search engine rankings, and traffic from those pages could be affected by the rewrite, you should consider leaving well enough alone. Rewriting your URLs may benefit you in the long term, but you could be creating more troubles than it's worth in the short term.
Google usually does a fine job following and giving credit through the 301 redirect process. However, some search engines (namely Yahoo) have issues following and giving proper credit and value when 301 permanent redirects are implemented. If your Web site's Yahoo traffic is significant, you could be gambling by redirecting. In this case, you may be better off leaving well enough alone.
I'm contemplating recommending URL rewrites for one of our client's Web sites. This client has enjoyed good rankings, but I think they'd get better rankings after the URLs are rewritten. This client's Web site has no more than two dynamic parameters within their URLs, but they don't have any keywords in the URL string.
The issue is that Yahoo is important for them. Their business is somewhat seasonal, and choosing a time to attempt the rewrites (and preparing for any possible short-term setbacks) is concerning.
For the first year I worked with this client, I chose not to rewrite the URLs. Their business grew. Their site achieved some good number-one rankings in the SERPs (define).
Should we leave this alone? Their business is growing. The Web site's search engine rankings are relatively solid (though there are always areas for improvement), and traffic and conversions are good. It's not going to be an easy decision.
Plan Ahead Before Implementing Any URL Rewrites
If you do decide to implement a URL-redirecting strategy, be sure to think things through before you begin making changes. Ideally, if you're making changes to your URL structure, you should consider moving subdirectories (anything following a slash) with keywords as close to the root domain (www.sitename.com) as possible.
For example, it's better to use www.sitename.com/KeywordA-KeywordB/ than www.sitename.com/Category-Name/Sub-Catagory-Name/KeywordA-KeywordB. Make sure this doesn't affect usability of your Web site, though. For SEO purposes alone, however, this is preferred.
Be sure to think things through ahead of time, if you do decide to make changes. I've seen it happen where after just recently completed one round of URL rewrites, someone in the organization doesn't like the way they were done, and they have to be re-worked again.
This will get you into a situation I call a "301 redirection loop." This happens when you've told the search engines that www.sitename.com/ukaseguagfug is to be redirected to www.sitename.com/Category/KeywordA-KeywordB/. A few weeks later, you determine that you'd like to rename the /Category/KeywordA-KeywordB/ page as /KeywordA-KeywordB/.
Now, what you're doing is 301 redirecting a URL that was just the recipient of a 301 redirect. This can raise a red flag with search engines, who get suspicious of a webmaster's intentions when multiple redirects are involved. Yahoo, in particular, will have a problem with this.
The Bottom Line
While URL rewriting can have SEO benefits, it can also cause more SEO problems if it's done hastily, or incorrectly. Also, there are more things than SEO to consider, such as usability and site architecture, before making the change.
Don't just jump into URL rewriting because "you want keywords in the URL string" or you read that this was what you need to do. Some Web sites do just fine with a couple of dynamic parameters in the URL string. But, if you receive good consultation from a good SEO professional, sometimes URL rewriting can be the ticket to achieving some pretty remarkable results!