Generally speaking, the bigger the better. Depending on your computer, resolutions past 600DPI may be a little slow to work with, but really it's common sense that higher quality scanning will lead to higher quality results in the end. Also, there is really no reason to align whatever you're scanning to the edge of the scanner - something will inevitably get cropped, and while a crooked scan can be fixed in 10 seconds, there's no way of regaining cropped parts of an image (short of manually redrawing it).