Jim Hamm sends this helpful advice: "Following is a posting (not mine) from a forum I subscribe to on using iCloud for backup. This may be helpful in determining what—and how much—of something you do want to backup to iCloud. Every app you have installed is listed and how much storage is used for that app. Storage can be turned on/off for each app."
"For those of you who will be installing iOS and using iCloud tomorrow -something it took me quite awhile to learn - if you have an app with a lot of content (iBooks for example) make sure you turn it OFF in iCloud backup (iCloud > Storage & Backup > Manage Storage > Backups / Devices / Selected Device >; Backup Options > iBooks/other App > Off). I sync a LOT of items to iBooks on both iPhone and iPad (1.9GB > 2.3GB) so I was filling up my 5GB of 'Free' space and was quite annoyed that backups were taking up space but once I realized what was taking up 4.2GB I turned iBooks off on both devices and my backups went down to 400MB & 500MB leaving me 4.1GB free for other storage."