Sgt_Kickaxe - 12:35 pm on Jan 25, 2013 (gmt 0)
The only preview I would consider blocking is in the preview panel of your CMS (e.g. wordpress preview). When you preview your page you ask adsense to load ads relevant to the url and Google never forgets a url even if is a preview=true url.
While I'm sure they deal with the issue because it's widespread I don't particularly want to be showing ads to myself or sharing preview urls with Google.