Just to add to this, we've had cases where moving from the bottom of the body to the top has improved tracking by approximately 10% on HTML pages with a lot of assets on them. If your pages are relatively small, chances are that the difference won't be as high as this particular case, but on larger/longer pages, it can be a bigger issue if utag.js is buried at the bottom.
Check out our new Swift integration library for iOS, macOS, tvOS and watchOS: https://github.com/Tealium/tealium-swift with updated
documentation https://community.tealiumiq.com/t5/Swift/tkb-p/swift.