OP,
Honestly the way I do wires is in phases. Speaker cables and interconnects and stop. I will use the 10% as a guide on those. [understand, I have all sorts of wires laying around from the past 50 years of audiophilia]. When I have finished, I will do additional moving speakers / room treatments. Then I will attack the power cords. By this stage I know my system really well and have recovered financially. So, I will then see how much effect power cords have. So, I guess I have always added those later outside of the rest of the budget.
However, the amplifier power cord can often make one of the largest impacts… up there with the speaker wires. If you make sure you get a big sonic impact from each purchase then if you end up spending more it is OK.