It's a decade later, and still pretty much the case... I'm a pretty heavy advocate for quantizing png files into a lower color palette combined with zopfli compression. The fewer colors on the palette the better.. note: not great for real images, but pretty great for lower color generated graphics, etc.

For 2-color scanned images, reducing the palette to 2, and zopfli combined is usually 1/7th the size of the original tiff or smaller as an optimized png. I've gotten similar results with many drawing based png as well.