It seems to me that there are two ways to back a sailboat. One is to get up enough speed in reverse so that the rudder becomes effective, and then use the rudder to steer the boat where you want it. The other way is to learn the direction and magnitude of prop walk for your boat, and line your boat up slightly cocked at the entrance to the slip, so that, as you back up, it will pull the stern backwards and to one side, and the boat will end up centered in your slip.
The problem with the first method is that, when the boat is in the slip, you have to use a shot of forward to stop momentum, and if your timing is off, you hit something. I see guys do it successfully all the time, but you only need to make one mistake to do damage.
The second method is slower, but much safer.