To begin, I should explain a handcuff for those first timers who may be reading this. A handcuff is the backup to your premier back, who you draft and keep on the end of your roster in case your star gets injured.
I think too much has been made of handcuffing without anyone ever explaining when you should do it. In standard leagues, you rarely need to use this technique. For example, if you have Steven Jackson, and he gets injured, you won't be plugging in his backup and expecting good results. You go to your bench or the wavier wire. There's no point in clogging up a bench spot with someone who might have value at some point when there are guys with real value there.
Typically, you would only handcuff in a deeper league, where there isn't much on the wire and you may not have a backup. In a 14 or 16 team league, you spend a lot of resources on your star and you need to have something if he goes down. Even in this case you still only want to handcuff if its a team will allow a backup to succeed.