While others have spoken against submitting your sitemap to Google et al., I am going to stand up for sitemaps all across the web, and give you several good reasons for having a sitemap (now you can do sitemap autodiscovery instead of submitting).
1) Get to know your site – Every site owner should know what a sitemap is and to have at least physically seen what it looks like and glance at the pages it contains. Many sitemaps, interestingly, end up with images, favicons, and stylesheets listed in them, which is not what the sitemap is intended for.
2) Improves index freshness – If you blog once or twice a day, run a news site, or have user-generated content, your sitemap can help search engines get familiar with your pace so their index keeps up nice and fresh.
3) Trust and Credibility – Although probably not a huge factor, submitting a sitemap does show to search engines you are real, exist, and care enough to create a sitemap. I doubt the majority of the spam sites that steal blog posts (mine and yours) for their own content actually use and submit a sitemap. My philosophy is that anything that can improve credibility and trust with a search engine is a good thing, especially when establishing a new domain.
Some people have complained about seeing pages drop out of the index because of submitting a sitemap, and then once they remove it they see an increase. There’s a nice thread over at digitalpoint on this topic. Ultimately I don’t think it’s necessarily your sitemap. The number of pages indexed can fluctuate week to week for any site. And if there is a huge drop in pages, you may want to consider checking out your sitemap to see if it is giving full coverage to all the pages of your site.
So my opinion is yes, you should submit a sitemap. Some people think the search engines are “evil”, but they’re really trying to deliver a fantastic product that users will enjoy and use. They’re not trying to “trick” us by asking for a sitemap. They have spent millions of dollars supporting the ability to submit a sitemap, and its not so they can drop pages from their index. They want to be better and they want you to be better.
So go ahead, create a sitemap!