To elaborate a bit:
The preview is not the graphic. When you choose 8 bit Mac or 8 bit TIFF, you are choosing the format of a crude, low-res thumbnail that is only there for your convenience when you place the image into a page layout program like QuarkXPress. You can choose no preview at all, and you'll just get a gray box when you place it in a page layout program--but it will still print as it should to a PostScript printer.
The TIFF preview can not contain transparency. That means in some programs it will seem to have a white box around it on your computer screen, though the white box will notprint.
The Mac preivew can't be used on a PC. If you put the image on a PC, the preview will disappear.