Mine are up high and work just fine, it's a matter of what's best for your available space. It's also more money for tubing down low, that silicone aint cheap.
I can't imagine you'd be using any pump where that slight of a difference in head would make any difference. Use whatever configuration suits your personal preference and placement of other components.
I thought I read somewhere that it's better to place the pump as close to the kettle as possible while still being lower. I can't find that post now but I could've sworn it was the guy from chuggerpumps that said that.
I'm probably gonna mount it closer to the kettles. I wanna save a little bit on the tubing.