Al final hice lo que no quería... Quedó como sigue:
Código HTML:
<table style="padding-bottom:2em;">
<tr>
<%If agno = 1 Then%><th>Año</th><%End If%>
<%If mes = 1 Then%><th>Mes</th><%End If%>
<%If region = 1 Then%><th>Región</th><%End If%>
<%If provincia = 1 Then%><th>Provincia</th><%End If%>
<%If comuna = 1 Then%><th>Comuna</th><%End If%>
</tr>
<tr><%If agno = 1 Then%>
<td>
<asp:DropDownList ID="DropDownList1"
runat="server"
AutoPostBack="true"
CausesValidation="false"
AppendDataBoundItems="true"
CssClass="select_variable">
<asp:ListItem Selected="True" Value="0">Seleccione Año...</asp:ListItem>
</asp:DropDownList>
</td>
<%End If%>
<%If mes = 1 Then%>
<td>
<asp:DropDownList ID="DropDownList2"
runat="server"
AppendDataBoundItems="true"
CssClass="select_variable">
<asp:ListItem Selected="True" Value="0">Todos los Meses...</asp:ListItem>
</asp:DropDownList>
</td>
<%End If%>
<%If region = 1 Then%>
<td>
<asp:DropDownList ID="DropDownList3"
runat="server"
AutoPostBack="true"
CausesValidation="false"
AppendDataBoundItems="true"
CssClass="select_variable">
<asp:ListItem Selected="True" Value="0">Todas las Regiones...</asp:ListItem>
</asp:DropDownList>
</td>
<%End If%>
<%If provincia = 1 Then%>
<td>
<asp:DropDownList ID="DropDownList4"
runat="server"
AutoPostBack="true"
CausesValidation="false"
AppendDataBoundItems="true"
CssClass="select_variable">
<asp:ListItem Selected="True" Value="0">Todas las Provincias...</asp:ListItem>
</asp:DropDownList>
</td>
<%End If%>
<%If comuna = 1 Then%>
<td>
<asp:DropDownList ID="DropDownList5"
runat="server"
CausesValidation="false"
AppendDataBoundItems="true"
CssClass="select_variable">
<asp:ListItem Selected="True" Value="0">Todas las Comunas...</asp:ListItem>
</asp:DropDownList>
</td>
<%End If%>
</tr>
</table> Gracias de todas maneras :D