Try adding this script to your grid question:
<script>
$(document).ready(function(){
$('#[% QuestionName() %]_div select').change(function(){
if ($(this).val()) {
var resps = [1, 2, 3, 4, 5];
var emptySelect;
$('#[% QuestionName() %]_div select').each(function(){
var value = $(this).val();
if (value) {
for (var i = 0; i < resps.length; i++) {
if (resps[i] == value) {
resps.splice(i, 1);
break;
}
}
}
else {
emptySelect = $(this);
}
});
if (resps.length == 1) {
$(emptySelect).val(resps[0]);
}
}
});
})
</script>