ciao!
si sono sicuro, anche perchè ho visto che il comportamento di default è quello.
cmq ho mezzo risolto così:
codice:
<script type="text/javascript" 
src="https://maps.googleapis.com/maps/api/js?key=API_KEY&sensor=true"></script>
<script src="http://www.mediwales.com/mapping/wp-content/themes/default/markerclusterer.js" type="text/javascript"></script>
<script>
    var centerMap;
    var map;
    var mapOptions;
    var min = .999999;
    var max = 1.000001;
    var cluster = [];
    var info = '';

    function initializeMap() {
        centerMap = new google.maps.LatLng(.........);
        var mapOptions = {
            center: centerMap,
            zoom: 2,
            mapTypeId: google.maps.MapTypeId.ROADMAP
        };
        var infoWindow = new google.maps.InfoWindow();

        map = new google.maps.Map(document.getElementById("map"), mapOptions);
        $.getJSON(..........., function (json) {
            $.each(json, function (i, item) {
                var offsetLat = item.lat * (Math.random() * (max - min) + min);
                var offsetLng = item.lon * (Math.random() * (max - min) + min);

                var marker = new google.maps.Marker({
                    position: new google.maps.LatLng(offsetLat, offsetLng),
                    map: map,
                    title: item.clie_ragsoc
                });

                var tooltip = '<div id="tooltip">' +
                        '<p>' + item.clie_cod + ' - ' + item.clie_ragsoc + '</p>' +
                        '<p>' + item.clie_citta + ' - ' + item.clie_prov + '</p>' +
                        '<p>PZ: ' + item.qta + '</p>'
                '</div>';

                google.maps.event.addListener(marker, 'click', function (i) {
                    info = tooltip;
                    infoWindow.setContent(tooltip);
                    infoWindow.open(map, marker);
                    $('#info').html(info);
                });
                cluster.push(marker);
            });

            var mc = new MarkerClusterer(map, cluster);
            google.maps.event.addListener(mc, 'clusterclick', function (cluster) {
                var cl = cluster.getMarkers();
                info = '';
                for (var i = 0; i < cl.length; i++) {
                    info += cl[i].title + '<br>';
                }
                $('#info').html(info);
            });
        });
    }
    google.maps.event.addDomListener(window, 'load', initializeMap);
</script>
ho usato MarkerClusterer.