onsubmit checkbox submission

I would like to update a WO page when a check box is ticked/unticked.

I have placed a WOCheckBox within a WOForm.

The WOCheckBox has onclick="this.form.submit()" and checked set to myVar
and the WOForm has action set to a callback method myCB

What I see is that setMyVar is only called when the check box is ticked, 
but not when it is unticked.

The myCB for the form is called for both ticks and unticks, but then this 
isn't passed the state of the check box

I can add some convoluted logic to work out the fact that the user has 
unticked (the fact that myCB is called without setMyVar having been called 
in the same cycle), but there must be a better solution?

Thanks
John