I know it is important to use the same url every time for search engines, but is it better to use the www or leave it off?

I always rewrite to the www version of the TLD.

I do this because people are used to seeing it there, and helps identify the text as an internet address at a glance in your marketing. Even if they don't remember the URL or visit the website, seeing "www" helps identify the business as legitimate.

For SEO, I don't believe anyone has evidence that the www serves better than the non-www... to your point, just be consistent and always rewrite to one of them.