I just happened upon a helpful key combination that can temporarily display a preview of a scan in Image Capture.
If you hold down Control and Option when viewing a scan Overview in Image Capture, the background changes to gray, and any rotation in a crop is applied. This makes it easy to avoid any stray white borders on your scans.
[crarko adds: I haven't tested this one; you don't get the same options on a camera import. I looked and didn't find a previous hint for this.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101208110603884