> Keep in mind that as images, pngs require a signal processing
> approach for optimal compression.
That's what I thought, until I did some experiments. My examples were map tiles, much like you'd get from OSM, Google Maps etc. It turned out that just gzipping the raw image data (24bpp) got slightly better results than with PNG's filtering. (And "no filtering" is not something that pngcrush tries, IIRC.) I then tried bzip2ing the raw image data and got even better results.
Less surprisingly, JPEG with a high quality setting gets visually-indistinguishable results and significantly smaller file sizes.
If you are motivated to improve compression by e.g. bandwidth bills, and especially if your data might be non-typical in some way, it seems that it is well worth experimenting and trying everything.