Hi,
SiteMap.aspx is for human use not google use. It has no content and it should not be indexed or crawled by google. In fact it could potentially hurt seo if it was crawled it might be seen as a link farm or having too many internal links. So there is no value to letting google crawl it and potential downsides to doing so.
For the correct urls for site maps that are built to be submitted to google see the article Submitting Your Site Maps to Google and Bing
Hope that helps,
Joe