Too large is better than too small, you can always resample it smaller. But no, the Best way is to know how to simply scan for the goal that you want to achieve.
Scan resolution just determines enlargement of the copy. But you can easily create far more pixels than you can use, and there's not much point of that.
The purpose of high resolution is for enlargement. It is not so much for more information in some nebulous manner, but specifically is to have sufficient information for the goal you want to achieve. So an important way to see it is this way:
There are generally two different goals for scanning an image, those goals being to show it on the video screen, or to print it on paper.
Print Resolution:
The ratio of (scanning resolution / printing resolution) is the enlargement factor.
For example,
Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (double size or 200% size)
Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (original size or 100% size)
Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (half size or 50% size)
If you are going to print original size, just always scan photos at 300 dpi.
Antique contact prints might have as much as 600 dpi of detail available, but our printers to print it are more like only 300 dpi. Common color photos don't have much more than 300 dpi of detail. There is little point of scanning a Walmart color print at 1200 dpi.
Film is typically small, and so needs more enlargement, and thus higher scan resolution for film.
One example is to scan 35mm film at 2700 dpi, and print at 300 dpi, for 2700/300 = 9X size enlargement. 9X is about 8x12 inches (about A4 size) from full frame 35 mm (about 0.9 x 1.4 inches).
The ratio of (scanning resolution / printing resolution) is the enlargement factor. You don't need and cannot use more than you need. Plug in your own numbers.
This is called "scaling", and this enlargement concept is true for scanning anything, photo prints, documents, film, etc.
Video Screen Resolution:
The role of scan resolution for the video screen is to create the appropriate image size in pixels. Images are dimensioned in pixels. For one example, scan 6x4 inches at 100 dpi, which will create
(6 inches x 100 dpi) x (4 inches x 100 dpi) = 600x400 pixels
Plug in the appropriate numbers to get the size you want from the size photo that you are scanning.
Your video screen size might be set to show say 1366x768 pixels (this varies, there are different size screens), and the concept is that this 600x400 pixel image will fill 600x400 pixels of that 1366x768 pixel screen (this example image width and height is near half of this full screen size dimensions).
That formula is true for printing goals too, meaning that if you will print 8x10 inches at 300 dpi, then of course in preparation, you need to create in the ballpark of
(8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400x3000 pixels
But again, (scan resolution / printing resolution) is the enlargement factor.
Example: If scanning 3.5 inches to print enlarged at 9 inches (at 300 dpi), then scan at 9/3.5 = 2.57 enlargement factor, or 2.57 x 300 dpi = 771 dpi.
Your scanner offers a set of common (favored) resolution numbers (like 100, 150, 300, 600 1200 dpi). I suggest using the next larger number that your scanner offers (as opposed to entering 771 dpi). It's not rocket science, I might use 600 dpi for 771 dpi, close enough. But scanners may do slightly better using their one of their set of numbers (matches the hardware better).
One issue is that prints don't enlarge very well. More than 2x is suspect quality. Film is designed to be enlarged, and does much better.
