CompMovieGuy: The phrase "diminishing returns" does not mean diminishing quality. It just means you are spending more and gaining less.

While it may improve the quality slightly by doing more passes, there is just a point where the improvements per pass are next to nothing.

I would not bother with more than 3-4 unless you are using a particularly low bitrate or trying to squeeze a lot onto one disc.

Frankly, I'm a fan of the one-pass vbr with prediction since your quantization is constant... but that's another conversation.