hawk
if you will permit an outsider to jump in ......
The only time one needs to use taper ratio these days would be in the very early project stages when one is estimating fairly basic aerodynamic characteristics. Once the design has progressed beyond the back of an envelope stage there are more sophisticated methods available that account for the "proper" geometry.
In any case, the basic aerodynamics of a wing don't vary all that much with taper ratio for the usual range, so the exact definition is, as Volume says, not Very important.