An interesting debate evolved today centered around tagging best practice, mainly in consideration about the rights or wrongs of tagging at the top or bottom of the page.
The top of the page has the obvious issue that you are beholden to the availability of your analytics provider, so unless you are brave then one to be avoided but in theory gets you the largest set of visitors tracked, even those that click away before the page loads which is arguably not valid, but is it when you consider you may have spent your PPC budget on acquiring them.
The bottom of the page carries less dependency risk if your analytics provider goes down, and is typically where most people would tag their pages.
The positioning of tagging on a page can make 20-30% difference on visitors you track. Also relevant is the fact that if the site is very slow, or uses a lot of display advertising then the visitor may have consumed, or scanned the content to make an action/decision before being tracked. Whether you count these visitors or not becomes quite relevant, especially as this could affect landing page metrics, individual page bounce rate and overall bounce rate.
The evidence is fairly anecdotal, but the moral to this one is be careful where you tag and really think about what purpose your site serves to consider the location of your tagging. Also consider each type of page, checkout confirmations are a great example of top tagging versus the rest of the site where you may deploy bottom tagging.
By top tagging, I mean anywhere after the <body> tag and bottom tagging, immediately prior to the </body>
