Many ask if they Should I go back and modify pages so that the spiders will see that there has been something new on each page? Or is adding more pages/post enough to tell the spiders that their sites are active and useful.
Well it is understood that Google determines an acceleration or deceleration trend in your sites “fresheness” level by how often pages are updated since the last time the Google bots came by your site. This is why you want to create interactive pages these days and definetly encourage comments on posts and pages because they will trigger new content added to the page.
But to be safe, you should be adding at least a response to a comment yourself to posts for adding a new paragraph or so of copy to keep them fresh at the rate you specified in your Google XML sitemap. You can determine how often you told Google you were going to update your pages and posts by clicking on Google Sitemaps under your main settings tab in your wordpress dashboard.
This is not to say you will get penailzed for not updating old pages frequently however. You can not become a slave to page changes. You have to be realistic. Just add new topic (keyword focused) pages and posts at a reasonable rate when you can. Always encourage viewers to comment on your posts and pages so all you need to do is moderate comments.
At least once a month you should be doing more keyword phrase research and be adding new unique, interesting content based on those new phrases. Your content should be at least 10 paragraphs in length and providing useful links with anchor text to other posts on your site also.
With the launch of Google’s indexing system “Google Caffeine”, you will get ranked almost immediately for new phrases as it will be a live index instead of a 5 layer cached index as it is now. So you want to be one of the first to have a new keyword phrase trend added to your site content.