We recommend installing fs.js in the <head> of your page, as high on the page as possible, for maximum data capture and playback fidelity.
When the script is further down in the page it may take a little bit longer for it to start capturing data depending upon whether there are blocking resources earlier in the page. Depending on how your site is set up and what other scripts you have running, you may miss a second or two at the beginning of your sessions. By installing the FullStory script as high as possible, you'll be able to capture all requests and calls on the page.
For more on how we load our script, check out this article.