Anywhere from 2 to 4 degrees is typical. I've had planes that needed as much as 7 degrees. The washers are a good idea to get started, but once you figure out how much you need, it's a good idea to sand a wedge from plywood and fuel proof it. Use the wedge to replace the washers.

There are a couple reasons. First, the washers will probably sink into the firewall over time which not only changes the thrust adjustment, but also loosens the engine mounting bolts.

Second, they create a stress point right around the holes in the engine mount. If the engine vibrates enough, it can cause the mount to crack eventually or immediately on a nose-over, for example.

A lot of guys use washers forever, but I think the wedge is a lot better.

In the image here you can see a thin wedge between the mount and the firewall. This plane didn't need much of a thrust adjustment.