There's no real guide about the quality "we want" here.
Because we'r not searching for the ultimate quality.
The principal goal is to be able to read scans (kanjis, liner notes...etc...)
The important limit is the file size the server is accepting. Which i think is in between 4 and 5 Mb for a single file.
Though i think i said a few times in the past, 3~4 Mb for a single page is a bit too much and such size should be ok for a "double-page".
Regarding on how scans should be scanned...
Once again we'r not forcing anyone to damage their booklets, so it's up to the user.
Though if you ask me how it looks the best, it's by flatening the booklet on the glass and take the scans normaly on "double-pages" without removing the staples.
For booklets that have more than 2 pages when unfolded, i suggest to have a look at
my other guide.