If you look at this process from a logical standpoint it is clear that it is not a onetime step. Since Search Engine Optimisation is not much more than the process of marketing a website online and it is something that is continued for the life of the business. Would you stop marketing your business because you felt like you had done enough to get some visibility? Most likely not and it is the same way you will operate with your Search Engine Optimisation efforts.
Search Engine Optimisation is a form of marketing that triggers a series of events online. You have to understand that ranking is one of those events out of a series of many other online marketing events. The ability to rank in the search results is not going to grow your entire business but it will help you creep along. Treating your website like a brand or business is going to allow your website to grow much stronger and quicker over time.