What i am understanding is ,you are hiding the label when page loads or before postback using javascript but same javascript is not working after postback.
if this is the case then keep your javascript inside the following add_endRequest function and check.
<script type="text/javascript">
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_endRequest(function () {
//put your javascript here to hide the label
});
</script>
Thanks
Happy coding