The aim should be to make the smallest possible file with adequate quality. The final printed document will come out at 300 dpi, so it is pointless scanning with more than this. For colour and greyscale, this means a maximum of 100 dpi because the printer cannot handle any more.
The final size of the image can also have an effect on the density because scaling down a scanned image in the document will concentrate the information i.e. reducing a 300 dpi image to half of its size will increase the density to 600 dpi.
The way to set up the scanner is:
Scanning images with dark backgrounds (like oscilloscope photographs or screen dumps) produces large files. By using an image inverter (usually an option on the scanner program) and using the negative, the information remains as clear but the file is dramatically smaller.
Photographs are very difficult to handle: there is an inevitable loss of resolution and the drawing the image on the screen is slow. If fine detail is not an issue, the results can be acceptable. Using compressed JPEG images (with e.g. Adobe Photoshop) is probably the best way to achieve high resolution with minimum file size.