It's a fair question, because *some* airlines (Paddington Bear hard stares at LH, LX, SN) vary their fuel surcharges based on fare class, and of course APD can vary based on cabin too.

I have usually found that doing a trial booking on ITA using C (for Award Business) and B (for Award Economy) gives me accurate tax figures. Certainly accurate enough to budget with, and to argue the toss with the ICC when their fantasmagorically calculated number is hundreds of pounds more.

The other benefit of using C and B is that they are Star aligned classes, so you should find them correctly mapped on all *A airlines. (Obviously, put ITA into the "don't check seats" mode and use the /f bc=C or /f bc=B operand to force it into the relevant fare class).