It's an interesting question...
You could also ask 'which section of the wing is used to calculate angle of attack?' as in a wing with washout (change in the angle of incidence from the root to the tip) the angle of attack will differ from root to tip. Even without washout, most planes don't have full length ailerons or flaps (or flaperons) so it will be rare that a wing has any single angle of attack.
Practically speaking, we're probably most interested in the angle of attack at the wingtips, because a stall here is most likely to lead to a stall/spin accident.