All SEO stands for is search engine optimization. Or in layman's terms, getting your website on the first page of Google, and hopefully in the first spot as well. Once you are sitting nice and pretty there, you are done. It is very rare for rankings to change dramatically except for when you first get ranked. After a couple of weeks of being on the first page of Google, you can safely assume you will be there for a while. It is also not recommended for a company that has just developed an internet presence, or one that has one online element and is trying to build their web presence/traffic.
It is always good to maintain your websites rank