Yes you do. Google won't crawl it as often as you post. Just keep in mind your limited with RSS feeds as sitemaps for forums because the max URLs u will have is 16 so you will need to update it as often as possible the best thing to do is use it and use the sitemap recommended by search engines
The following URLs have free sitemap generating tools http://www.xml-sitemaps.com/ www.freesitemapgenerator.com www.writemaps.com Of which the first one is considered as the best