When advertising your website to potential customers or existing clients, there is a difference in how to do reference your domain on a printed ad (like a mailed post card or sign) vs. an email or link on another website.
For print, keep the website reference as short as possible.
When listing it online, always use your primary domain for SEO purposes.
Anytime you link to your website on something electronic (emails, other websites, advertisements, forums, etc), always, always, ALWAYS use your primary domain. If you have chosen your primary domain to have "www", then be sure to include it in the link.
This helps Google give you more "SEO Points" (as I call them), meaning they'll include these inbound links when deciding how highly to rank your website in their search results. If you don't use your primary domain for the link, Google won't know to give you any credit for the link!
If your domain is lengthy and you don't have room for the entire domain to display, there's still hope! You can set the link text (the blue underlined text that is displayed) to be something shorter (like your agency name), and the link that's "behind" the text to your primary domain name. For example, Insurance Agency Website is a link to our website page.
If your printing a postcard or sign or any type of offline advertisement, feel free to use the shortest version of your domain that you have registered. Google can't see these ads, so you're not missing out on any SEO opportunity by using an alternate version of your domain. Just be sure to have completed the above recommendations and have that domain redirected to your primary domain.