Voltage is not the concern here, what it is whilst idling/under no load will be VERY different to that shown when under high CURRENT(amps) if all is not well wiring wise, where it could drop markedly.
The reason you don't take to the body/close point although easier is resistance, the more connection points you can eliminate between the pump and battery the better it will be, it will also NOT be prone to any corrosion issues, that could be encountered.
I have seen them earthed to the spare wheel well with a self tapper before now though, it's your car of course so up to you, but can't understand 1/2 a job with you doing the live, the earth is just as important, and even easier.
tabetha