From what I have seen and heard, the bobbin washers help if you use a bobbin that is a bit shorter than your machine needs.
For instance, many of the Janome's take a Class 15 bobbin which is 3/8" (or 6/16") tall and a lot of the prewounds are Class L which are 5/16" tall, so putting a bobbin washer in when using the shorter bobbin helps stabilize the bobbin. If you have a machine that uses the Class L bobbins, you generally can't use the Class 15 because they are just a smidgen too tall. If the bobbin is the size the manufacturer recommends, I don't know if you would see much difference.