TaxInclusiveTotalAmount and TaxExclusiveTotalAmount

Folks

Something that occurred to me looking at totals on an invoice.

We have three totals in LegalTotal, quite logically to cover
before and after discount and after tax.

But what if I'm not registered to charge tax in my invoice?! 
(I think that covers much of USA especially but also quite 
a lot of SME's.)

The answer I'd prefer, though not satisfactory really (especially
since TaxInclusiveTotalAmount is the mandatory one) is that
you have to put the same amount in TaxExclusiveTotalAmount
and in TaxInclusiveTotalAmount and hope that nobody thinks
this means you are tax registered but just not including tax in
this particular invoice.

Any thoughts?

Steve