What does this option do?

The JSHint immed option is used to enforce the wrapping of immediately invoked function expressions in a pair of parentheses. This prevents the less common use of other operators to force a function declaration to be parsed as a function expression and ensures functions that would already be parsed as an expression to be wrapped in parentheses anyway to make the intentions clearer to the reader.

In the following example the function is parsed as an expression as it's part of a variable statement. It is not necessary to wrap the function in parentheses to be able to invoke it immediately:

/*jshint immed: true */
var MyModule = function () {
    return {
        someData: 1
    };
}();

Be aware that this option does not enforce the position of the wrapping parentheses. Some coding conventions specify that the closing wrapping parenthesis should appear before the invoking pair and others say it should come after.

When should I use this option?

The use of the immed JSHint option will cause a "Wrap an immediate function invocation in parentheses" error any time it encounters an immediately invoked function expression that is not wrapped in parentheses. It's common convention to place an IIFE within parentheses because it makes your intentions immediately clear to readers of your code. If you glance at the example above you would most likely assume that x refers to the function itself rather than the return value of it. By wrapping the function in parentheses you're less likely to make that incorrect assumption:

/*jshint immed: true */
var MyModule = (function () {
    return {
        someData: 1
    };
}());

Note that this is an enforcing option which means JSHint does not apply it by default. If you do not explicitly set this option to true JSHint will allow IIFEs without wrapping parentheses.

Recommendation

Set this option to true (enforces the wrapping of IIFEs).