You should secure it fully forward, so it will not have anywhere to go should you have to break heavily...
It should also be secured as you are responsible for anything flying out of your vehicle... even if it turns upside down... Almost no one secures a load like that but most DOT regs require it... KennyV
I have this vision of rear-ending someone, the back flipping up, the mower flying out and crushing a bus full of elderly nuns taking hemophiliac children to a Make a Wish event...