Setting the position option will force this, so just use the same selector covering all your dialogs where I use #dialog here (if it doesn't find them no action is taken, like all jQuery):

jQuery UI before 1.10

$(window).resize(function() {
    $("#dialog").dialog("option", "position", "center");
});

jQuery UI 1.10 or higher

$(window).resize(function() {
    $("#dialog").dialog("option", "position", {my: "center", at: "center", of: window});
});

Here's that same jQuery UI demo page adding only the code above, we're just adding a handler to the window's resize event with .resize(), so it triggers the re-center at the appropriate time. ​