Normally I say, let it look ugly in IE6... however I had a page where some icons were pngs (with alpha trans) and the background for them was black in IE6 instead of grey. This made the icon unreadable yet still showed no alt text which meant functionality was broken... so I did the old "transparent.gif" with the png as a background image, so I could send one to IE6 and the other to the rest.
PNGs for some reason are considered to have a "colour profile" in my retarded Firefox even though I do NOT set one... this has been sending me back to .gif when I'd otherwise use .png : ( I have no profile to remove, yet FF does gamma "correction" anyway. ARG! FF DIE DIE DIE.
Safari does it too but I have not found a single instance of Safari actually making my page look bad, so it seems limited to FF.