Posting as a solution here so others can use it.
function ShowRBL6() {
if (document.getElementById('<% = MyCheckBox.ClientID %>').checked == true)
document.getElementById('<% = RBL6.ClientID %>').style.display = "block";
else
document.getElementById('<% = RBL6.ClientID %>').style.display = "none";
}
so the next click will be also for unchecking will be handled.