So you could leave the axle set all the way back if you want. This way the axle is always straight, No need to try to keep the wheel centered as you're tightening axle nuts. And if ya hit a good bump at 35mph you KNOW your axle is NOT going to slip in the dropouts. If it does slip you're gonna loose all the alignment on both sides, pedal and motor. Since I've been doing it this way I've had no problems with slippage or alignment.
fatdaddy.
IT'S YOUR BIKE, BUILD IT YOUR WAY.
P.S. It'll also leave just a little more room for the box.