My thumb rule is no extension if the script goes to the local bin folder and `.sh` otherwise. Beyond syntax highlighting, the extension also helps for wildcard matching for file operations (`ls`, `cp`, `for` loop, etc).
And this rule has been followed the majority(if not all) interpreted and scripting languages. The likes of Ruby, python and JS have multiple examples. Whatever executable is in your $PATH it won’t have an extension.
Not sure if this convention is actually documented anywhere.
