My company offers quite a unique product, typically by invitation only. I don't think search engine traffic is going to be important to us. Do I still need to submit my website to the search engines?
No one is ever required to submit their site to the search engines. It's an entirely optional task. It is however unusual for an commercial organisation to turn away potential new business, which is what search engine traffic is. If someone has found your website as a result of a search they did on Google, the chances are they have some interest in what you have to offer. If you're getting inappropriate or irrelevant enquiries from web users who found your site through the search engines, then you need to give more consideration to the content of your website because your message isn't clear.
If you actually don't want any search engine traffic at all, then use a robots.txt file in the root of your website to disallow your site from being crawled or indexed. You can disallow crawling and/or indexing of any part of your site in this way.
e.g.
User-agent: *
Disallow: /
The above instructs all search engine crawlers not to crawl any part of your website. Not all crawlers will obey the instruction, but the major search engines will; Google, Yahoo, etc.
If your website is just for fun; perhaps a family website or something for a hobby you pursue, then perhaps you will have no interest in search engine traffic either way. Search engine submission in this case would definitely not be a productive way to spend either your money or your time. The crawler search engines like Google will more than likely find your site and index it in time anyway.