While I've never seen this on a customs form, or been asked this at customs, it used to be super popular at checkin. I never lie, so this would happen:
Did you pack your bags yourself? Yes.
Have the bags been out of your sight or control since you packed them? Yes, I left them at the hotel bag drop all day, or the conference bag drop all day, or they were in the trunk of the cab all the way to the airport.
Nothing ever happened to me from saying Yes that way. In fact, the question got way more complicated after a while (Have these bags been out of your sight and control in a public place or somewhere unsupervised?) before they just stopped asking it.
I have seen, on Border Security, that this question gets asked in by customs and immigration in secondary inspection. (Secondary is not the booths you line up to show your passport, but the room off to the side that only a few passengers are sent to.) Once they've decided to look through your bags, they may ask, in a super bored "just a routine question" voice: "Is this your bag? Did you pack it yourself? Are you aware of all its contents?" Then when they find something, people often say "What is that? I didn't put that in there!" and the agents are like "you told me you packed your own bag. Were you lying? Who packed this bag?" - it seems like it's a strategy to either forestall the "I didn't put that there" defense or to get you upset enough that you will tell them who you're carrying contraband for. I don't get the feeling it's a screening question, it's a question they ask once they decide to search you.
If someone asks "did you pack this bag yourself" do not EVER just say "No". Say "my girlfriend put all the clothes in for both of us" or "my mother is really good at filling suitcases so she put everything in after I chose what I was taking" or "the two of us did the packing". Just saying No sounds to the customs guy like "this guy I met a while ago stopped off with a full suitcase I never bothered looking in" and you might guess that's not a smart thing to do.
Don't lie though. There is nothing wrong with carrying something for somebody else if you know what it is and it's legal to carry. There is a ton wrong with lying even if the thing you're lying about isn't wrong. And these people, their whole job is spotting that you're lying. Don't even bother.