depending on how the image was scanned, you may lose considerable information converting to monochrome.
the safe setting for transparent backgrounds is 16 grayscale. usually you can scale down to 4 gray for text scans, or 8 gray for illustrations, with only negligible loss. it is not worth going to monochrome if you notice any image degradation. in those situations, it’s possible to force monochrome after fiddling with the color curve or gamma but it’s usually not worth it since you won’t shave off much file size compared to 4 gray.
finally, I recommend using PNG Gauntlet which uses PNGout for lossless compression. it can easily cut away 10 percent of the file size for PNGs outputted by Comic Enhancer, and up to 60 percent for Photoshopped PNGs!