Yes - putting on head can measure DOM loading and interactions as quick as they happens. But have performance hit - Webkit (Safari, Chrome, etc) doesn't show even single pixel on screen until they load all resources in head. And another bad news - HTTP partition cache for Safari and Chrome.
Putting in body - you can not catch all interactions, but won't stop rendering.
First is when GA script is on <head> section. This is mostly popular, but making CWV scores little bit low.
Second is when GA script is anywhere on page, but not on <head>. Like before </body> or in <body>. This doesn't hurt your CWV scores.