Power steering: more responsive, less stiff in reverse?

The only reason you'd feel any difference at all is because when backing up, your front steering wheels are trailing instead of fighting a forward thrust directly onto it like when going forward.

 

Cars that have variable ratio power steering often work like that as they go to max. assist in reverse.
Don't know which, if any, Prii are supposed to work like that though.

 

It's a function of the caster and the generally less stable dynamics of steering while going backward.
I've noticed this in my other cars too (including some without power steering).

 

Also, while in reverse you are only using electric power, limited to approx. 40 HP peak, though the available torque is pretty high.

 

never noticed it myself, but i only reverse to leave my driveway and parking spots typically.