Until recently, I didn't itemize on the back either. But a couple of times recently, when the officer asks what I bought, I tell him and he writes it on the back for me. So I've gotten so I itemize every time.
If you're like me, you always "estimate high" on how much you spent abroad. I can never remember exactly how much I spent abroad, so I'll put $200 or something and then write on the back:
$100 clothes
$50 chocolates
$35 music CDs
$15 misc.
That kind of thing. I figure that if they really care, I could pull out receipts, but I'm not going to itemize and add up every last $35.27 charge. Better to just say $200 and call it good. That way you aren't violating any laws.
Remember, always tell the truth on your declaration forms and in your conversations with the officers.