Definitively the best place to place the CoBrowser script is at the end of your web page, just before </body></html> tags.
This way your web page will load and render without extra waits.
We'd like to mention that our script uses async loading methods to speed up the load. It specifies that the script will be executed asynchronously as soon as it is available.
That means that the script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing).