neit:
According to google it should go before the closing </head> tag though I have seen other sites place it at the bottom of the
page before the closing body tags as you have said.
It seems to me it won't matter where it is as long as its picked up and sends data back to google (I may be wrong on that though). Which I would expect it to do
if its only appears on the checkout_succes page which would load completely after customers purchase.
I have put it in to see how it goes, aslong as it does not interfere with the shopping cart checkout functionality then I will try it to see what happens.
Seemed to be ok when I put through a test purchase. I can always delete the goal if does not workout as expected.
Used to be you put it on the bottom of your page so as not to slow down the rendering of the page as the javascript loads. However the new code Google uses loads the javascript asynchronously so this isn't a problem.
Best place to put it is probably in includes/templates/YOUR_TEMPLATE/common/tpl_header.php or tpl_main_page.php whichever floats your boat.
I don't see any reason why you would ONLY track the checkout_success page. Track everything.. then you can drill down to which hits were on what page.
After you get basic analytics working then work on getting a goal funnel for your e-commerce conversions.
You might want to look into the google analytics modules anyway. The one I'm using was actually quite simple. And it was easier to manage then my previous code hacks.