The commonly recommended way to use text item delimiters is like this:
set tid to AppleScript\'s text item delimiters
set AppleScript\'s text item delimiters
If you run a script application, then I guess technically it is not necessary to rest them, because once it has quit, it has quit!
But if you are running the script in Script Editor, then the changes to delimiters will stay in that script in the script editor. I've just seen that today.
But what changed some years back is that it does NOT affect other scripts / windows open in Script Editor.
Also, when running as an application, it only affects that particular script.