I put the +1 button on my site today.
As a matter of course if the external javascript code is asynchronous (google analaytics / facebook SDK etc.) I would recommend putting it very near the <head> section, or the top of the <body> section (somewhere called on every page of your site) so it is one of the first things to start loading. External asynchronous javascript can be called while your page still continues to load, it does not slow it down either - the user can even begin to scroll down as the content appears.
Non asynchronous javascript code on the other hand needs to load before anything else on the page, (when it is called). Nothing can render on the screen until this code has been called by the browser, which is why if the external server is under strain your page can 'hang', typical of non-asynchronous code from facebook or twitter.
This code therefore should go in your <footer> (somewhere at the end called on every page of your site) or at the very end of your <body> section of page code. This will allow your page content to render before needing to call this type of external javascript.
Google have recommended putting this non-asynchronous code for the +1 button at the start of your page code (at the top) near or in the <head> my recommendation is to do the reverse, put it in the footer or near the end of the code. The button will appear as one of the last things on your page, but importantly it will not interfere with your content loading - which the user is most interested in seeing first.
You may disagree but I found this helps my pages both 'load' and 'appear' to load quicker ... in essence:-
1. asynchronous javascript code high up in the page code <header>
2. non-asynchronous javascript code low down in the page code <footer>
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In this case put this code in your footer:- <script type="text/javascript" src="http://apis.google.com/js/plusone.js"></script>
and this code where you want the button to appear: - <g:plusone size="medium"></g:plusone> note:change the "medium" to "small" or "tall" or delete it, for whatever button you want etc.
