Two opposite answers...lol
from another forum thread, and this
one from google's site itself.
One thing to mention, I use some software called ZoomIndex, which is well an indexing/search feature for several of my sites. I have it scheduled so that is indexes my site on a weekly basis. One of the added benefits of this software though is that it creates a sitemap for me as well. So each time it runs and indexes my site, the sitemap is also recreated and saved over the previous sitemap.