A decoupling capacitor in the positive supply rail will source shortterm current peaks from the ground rail, in the negative supply rail it will sink shortterm peaks from the ground rail. The positive supply rail is a bad place to sink currents to, the negative supply rail is a bad place to source currents from.
The ground plane tends to be a better place to both source and sink current spikes to.
The seminal phrase here is "ground plane" as part of the power supply paths. Decoupling capacitors that are connected to the signal reference ground traces will add unwanted crossover distortion via the input.