The same question has been asked in accordance to a graphic designer, and the same answer applies really. The majority of website developers will obviously know how to code a website, and will understand the general practices required for search engine optimisation to be successful i.e. page titles, header tags etc. However, depending on your preferences and requirements will depend on whether to employ a professional SEO company to undergo your website marketing. As I have mentioned many times, there is no point having the best built website in the world if no one ever visits the website.
It is all too common nowadays that website owners get a site developed by a respectable website development company, who add-on SEO services. However, in the majority of cases web developers are clueless about SEO beyond the most common basics; so if you are looking for just the basic requirements and limited success then getting your website developer to handle the SEO is a logical step. However if you are serious about improving your website, then you are better going with a SEO specialist to deliver your campaigns.