Choosing the dpi to scan with is crucial. Detail, image size and file size are the most important things to consider when it comes to it. And you should always get the right dpi before the first scan to avoid making mistakes and being forced to start all over again. It's wrong to choose a low dpi just to save time as the results are poor. If you have the original film negatives, you should use them instead of prints. High resolutions capture highly detailed sources. It's always better to scan too high than too low.
Saratoga Scrapbook Scanning Will Provide You The Best DPI for Your Photos
Saratoga Scrapbook Scanning Will Provide You The Best DPI for Your Photos
Source : http://scanyourentirelife.com/dpi-should-be-scanning-your-paper-photographs/