I have had almost all of these behaviors toward backups. Until my first hard disk crash, I hadn't really thought about what could be backups
Therefore the first one was a disaster 
Then I adopted the first option as my moto: all data duplicated all the time. With the slow mediums of those old times, I was backuping my data all the time 
Then I gradually fell to the last option with the years passing, until a new hard disk crash! Those monsters seem to show up each time I lower my guard 
Therefore I settled for an intermediary solution that I kept over the years. And for the first time, my backups were actually usefull I wouldn't go as far as saying it was a pleasure to have a hard disk crash, but it certainly makes things more easy to accept 
These days, I do complete backups about once a month. However, I have intermediary backups. For example, when I have finished something important, I backup the appropriate directory, in order to have a copy on a different medium (in case of a hard disk crash for example). Once a month, I gather all my intermediary backups (that have been made up to date), consolidate them in a single file that I dispatch on 3 different mediums, as well as sending it on a server on the Internet (so even if my house burns, I still have a not too old backup available).
This however is only about the essential data, "only" a few hundred megabytes. Less essential data amount to several hundred gigabytes. That's why I had added a step in my backups: copy any file that has changed to a dedicated hard disk when doing the main backups. Thoses are the backups that were usefull to me about a week ago.
Knock on wood! 
