I came across an article on this somewhere yesterday. Sorry can't remember where. Apparently the driver can only rate immediately after the ride and before the next one he takes. The rider can rate at any time (or days/weeks later). The rider can also find out his rating before and after the trip. It was actually an Uber driver that pointed this out, but if the driver gives the rider a low rating, he puts himself at risk of the rider retaliating with a 1* which can be much worse for the driver than a rider.