That's like asking "is breathing clean air bad for you?" or "is dihydrogen monoxide (it's far better known as water) really deadly when consumed?" (Believe it or not, one city in California actually went so far as to try and ban dihydrogen monoxide on the grounds that it was a public health hazard until someone finally figured out what this "deadly chemical" really was.) The answer is no. Think about it. Google even says to submit a site map when you submit your site to their search engine. If they didn't want you using it, they'd flat out say so. Not only that, but site maps also help tell the search engines where your pages are so they CAN be indexed. It's really especially beneficial to those sites that are rather large and have a lot of buried content deep within their site.
A sitemap is important. I create sitemaps for all my sites. To use Google webmaster tools effectively you need a xml sitemap
Note I was talking specifically about the XML file (which is why I said no about the Web site optimization, rather than the search engine optimization) instead of an actual sitemap page (which is where I would have said yes, on the grounds of usability and accessibility).