In a portable, all grounds are connected together to form a contiguous path of return. This means you can ground perpherals like the screen, GC+, or audio amp basically anywhere that is convenient for your build. There isn't really a right or wrong location.
The same is true for voltage. You can tap it from anywhere along the voltage path to power whatever you need, so long as the wire/trace is thick enough. You don't need to run a wire directly from the regulators to each component, you can daisy chain matching voltage locations from board to board instead. Typically, people choose to run power and ground to the Wii and then use the Wii's many voltage and ground locations as a free breakout to power perpherals.