The important thing is that the ads look different from the content of your blog. Here is what the official Google Adsense Blog said: http://adsense.blogspot.com/2008/03/another-look-at-optimizations.html Click the link to see an example. You can see some other examples of what not to do from the Google Adsense help files: "placing ads under a misleading heading" https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=136881 "encouraging accidental clicks": https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=136901
No, it is not. You just need to make sure visitors are able to differentiate it as an ad, rather than part of your content.