I just read my Internet Genealogy/Family Chronicle Newsletter Vol 2 #1 (29 Nov 2012: Moorshead Magazines Ltd.) online and found this wonderful scanning tip:
Scanning lots of documents for your family history project? If so, scan them in gray scale, not color, and use a low-resolution of 72 dots per inch. This will save space and make your scans go faster.
However, if you are scanning images for the purpose of creating a report, or publishing a family history, then scanning in color or gray scale at 300 dots per inch resolution, will give you the best quality.
Scanning has become so popular for saving documents that we find in books and online. We save them to our genealogy programs but, the question is always, what scale should I use? For reprints, as suggested above, we need to use a higher resolution, but that also takes up hard drive space. Copies of censuses, city directories, maps, pension records, etc. can be rescanned later if needed for publication, so save that space on your hard drive and start scanning smaller.