jQuery - SmartWizard. How can I override "Previous" button validation?

Basically, I need to validate data when the "Next" button is clicked, but if the user wants to go back I do not want to validate if they have introduced the required fields for the current step

    $('#wizard').smartWizard({ onLeaveStep: leaveAStepCallback,
            onFinish: onFinishCallback
        });

        function leaveAStepCallback(obj) {
            var step_num = obj.attr('rel'); // get the current step number
            return validateSteps(step_num); // return false to stay on step and true to continue navigation 
        }

        function onFinishCallback() {
            if (validateAllSteps()) {
                $('form').submit();
            }
        }

        // Your Step validation logic
        function validateSteps(stepnumber) {
            var isStepValid = true;

            if (stepnumber == 1) {
                var e = document.getElementById("ContentPlaceHolder1_DropDownCustomers");
                var strCustomer = e.options[e.selectedIndex].value;
                if (strCustomer == "-1") {
                    //alert("Please select a Customer.");
                    $('#wizard').smartWizard('showMessage', 'Please select a Customer.');
                    isStepValid = false;
                    return isStepValid;
                }
                else {
                    var d = document.getElementById("ContentPlaceHolder1_DropDownTemplates");
                    var strTemplate = d.options[d.selectedIndex].value;
                    if (strTemplate == "-1") {
                        alert("Please select a Template.");
                        isStepValid = false;
                        return isStepValid;
                    }
                    else {
                        return isStepValid;
                    }
                    return isStepValid;
                }
            }

            if (stepnumber == 2) {

                if (document.getElementById("ContentPlaceHolder1_LabelMainDestData") != null) {
                    isStepValid = true;
                }
                else {
                    alert("Please introduce the Main Destination.");
                    isStepValid = false;
                }


                return isStepValid;

            }

            if (stepnumber == 3) {
                isStepValid = true;
                return isStepValid;
            }

        }

in showStep function just edit

if(stepIdx != curStepIdx)

to

if(stepIdx > curStepIdx)

it will resolve your problem