jQuery validate - set conditional rules based on user selection

jQuery validate with condition as a function result (adding on to Ryley's comment above):

$("#Form_Id").validate({
    rules: {
        CompletedBy: "required", //straight forward validation of field
        Contact_No: "required",
        Location: { required: noTableRow }//if no table row, make Location mandatory
    }        
});

//checks whether a table row exists
function noTableRow() {
  return $("tr.tr_class").length == 0;
}

You can use a custom validation method. For example:

jQuery.validator.addMethod("checkA", function(value, element) {
    return YourValidationCode; // return true if field is ok or should be ignored
}, "Please complete A");

then use it in your rules:

rules: {
    ....
    field_a: {
        required: false,
        checkA: true
    },
    ....
}

Take a look: http://jqueryvalidation.org/jQuery.validator.addMethod/

jQuery Validate actually directly supports this, using dependency expressions.

All you need to do is change your validate options like this:

$('#myform').validate({
    rules: {
        fieldA: {
           required:'#checkA:checked'
        }
    }
});

That's it!