Dan,
Depends whether or not you want the scanned image to be plotted. I find .tif's to be easier to work with when it come to "fading" the image on the plot. My default plot setting is 'draft' or 'fast' quality (for speed in plotting), so .jpg's plot very poorly with my typical plot setting.
Any raster image inserted into an AutoCAD drawing for "tracing" should be scaled very carefully. If the image was not scanned from an original on a flatbed scanner, the "X" & "Y" scaling will not likely be equal. I have found this to be quite prevalent when scanning copies made from wide-format copiers and the "ancient" diazo blue-lines.
Ralph
Structures Consulting
Northeast USA