I found something very important if by any chance you're changing the Mario sprite to something custom. Almost all of the Mario Sprite is located in GFX32, but there are small little tiles (such as the end of Mario's hand in P-Flight and his feet while swimming) that are in the GFX00 file right after the "Nintendo Presents" logo. It took me hours to find those missing pieces while doing this myself, and I figured to share the info.
I actually have in my work space a folder which contains screenshots of what each graphic looks like in YYCHR. So I just scroll through a bunch of images and look for what I need. However this is also very useful.