Most consumer-level scanners scan at either a 300 or 600 dpi optical resolution. Naturally, you might think that to get the best picture possible, you should scan at the highest possible resolution. If you're printing the image on a 720 dpi inkjet printer, you might even need to enter the interpolated resolution zone. Good thinking, and it has a nice logical ring to it, but that's not the way scanning works. These are actually two different kinds of dpi.
Most color printers have three or four different color cartridges.
This means that each dot that comes out of your printer can only be one of three or four colors. So what this means is that the resolution of your scanned image can be much lower than the dpi capabilities of your color printer and still produce the same results. Try it yourself and see. Maybe you're wondering what the problem is in scanning at too high a resolution. The answer is file size.