That depends on you.
If you run a company where customers from the internet make up a large part of your business then it is and you should be continually working on SEO even when you are at the top. If you don't one of your competitors might topple you from the search engine rankings with catastrophic results for your bottom line.
But if you run a car repair workshop and new customers are beating a path to your door because word-of-mouth is telling everyone about your excellent service then SEO is less important. They will find you because of you, not because you are number one on Bing or Google.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.