Rating the output of a power supply
One factor to consider is the power dissipation in the regulator, and the resulting temperature rise.  Even though the transformer / rectifier can supply the same current at 9V regulator output as it can at 18V output, the regulator power dissipation will be greater when the output is 9V (power dissipation = (Vin - Vout) * current).  Make sure that your regulator will not get too hot when the output is 9V and all your loads are connected.  Then the regulator temperature will be fine when you have 18V out.