You said "blog" - google scans blog RSS feeds to locate new content quickly, often adding it to the google blog search instantly.

For blogs a sitemap may be redundant since the purpose of "reporting new pages to google" is defeated by blog search.

Then again, even the adsense bot sometimes beats the blog search bots to the punch. Google has no shortage of ways of finding your new content.

By all means, sitemap your new articles, there is a chance that the standard "webmaster central" bot is sent out to your new article first after checking your sitemap. Since adsense and blog search (via your RSS) find new content you can't go wrong by keeping an up to date sitemap too if for no other reason but to keep things uniform.