What should wheel alignment be for best MPG?

Back when most cars had positive camber (front wheels leaned out at top), we heard that to avoid tread wear we needed toe-in to compensate for the wheel's tendency to roll in a curve toward where it was leaning. Now we have severe negative camber (Prius rear wheels: neg. 1.5 degrees, plus or minus 0.5). This suggests that we should have toe-out to compensate. Instead, Prius specs call for slight toe-in. Might this explain the many complaints of excessive tread wear? Has anyone out there experimented with shimming the rear wheels for toe-out? If so, how much toe-out would be appropriate for the specificed camber?