Scanning Technique
I usually leave "unsharp mask" disabled because that feature modifies the image from the original and once it's modified there is no going back. But it can be a useful feature for highlighting very subtle detail that would require a much higher resolution scan.
I used a pair of 9 cent bill stamps from plate positions 1-2 to demonstrate. There is subtle doubling in the top left corner, and the bottom frame line, and other marks in much of the lettering.
The links below show blow-up scans, beginning with a 1200 DPI image that has not been "enhanced". Then unsharp mask was applied using GIMP. The third shows a 4800 DPI scan for improved resolution, and the fourth is the same image with unsharp mask applied. The final image is based on the 4800 DPI image, but with unsharp mask applied, and the image scaled down to 1200 DPI.
Image at 1200 DPI (baseline image)
Image at 1200 DPI sharpened (sharpened)
Image at 4800 DPI (16x detail over 1200 DPI)
Image at 4800 DPI sharpened (sharpened)
Final enhanced image (previous image reduced to 1200 DPI)
The sharpened images show the doubled frame lines better, but the effect is artificial and it increases the file size. The 4800 DPI scan is ideal, however the file size is near the limit of what is acceptable to send over the internet. The sharpened 4800 DPI image is almost double the size of the unsharpened image due to strain on JPEG compression. The "final enhanced image" appears to provide a good balance of showing subtle detail and minimizing file size.