I'm sorry, let me rephrase the question. Should the user-friendly, hand-coded, site-map.htm page which only shows the top level navigation pages mostly. (as opposed to the ugly, auto-generated, automatically submitted to G, search-engine-targeted, sitemap.xml.gz page containing every page on the site, which we don't link at all because it is not that useful to users) be LINKED FROM EVERY other page on the site, or just say, from the top level or two level navigation pages which it links out to.
In the former instance (linked FROM EVERY page) it is definitely more convenient for the reader to find and get to, but you are sending a LOT of link PR to one central sitemap page which yes, will redistribute some of it back to the top level site pages it links, but in and of it's self will tend to get indexed and rank very highly for what? The oh so useful and niche-applicable term "site map" which you probably anchor-text link it with from each page for clarity :)? Even if you link it as "Brown Widget Website Site Map" that dilutes your anchor text (in highly competitive niches) to the point of mostly being found only for the term "Brown Widget Website Site Map" which no one is going to search for.