re: straightness
To retain the most integrity to an image, I want the
least amount of post-scanning file manipulation. I want to minimize any need for straightening and cropping whenever possible. One of the problems with scanner is that lifting the lid to readjust the positioning often causes the item to shift. Thus I start with an item, a partial sheet of contrasting construction paper and a few bean bags

I place the item near the center of the glass, cover it with the construction paper, and enough bean bags to hold it all flat:

A preview scan is made. Oops! Needs work.

The entire stack can be shifted slightly to square it up, make new previews of the ajdustments, and then a final scan at a resolution to make a file somewhat under 300k for posting here or
ebay or wherever.

The small extra border helps define the edges and corners which are important to evaluating condition, etc. And in this case, the cover is not quite square, so a small border is both necessary and desirable. Try to make a "square" scan of a stampless folded letter!! Pick an edge and go with it.
If I'm scanning for a listing on
ebay, I eyeball it once and make the scan. It won't matter if its off by a few degrees. Time is worth more than a perfect image.