I've had a bag searched at NRT. Had quite a lot of luggage, got asked about it, and what they saw by going through the largest bag corroborated the information I had given (they didn't need to continue with the rest).

Also, don't agree that addresses are useless to foreigners. Picked up the Kodansha Tokyo Atlas for my first visit (2002) and now it's even easier to find anywhere (and see how the address system works) using Google Maps.
Seems to me that having a cousin in Japan just made the OP rather complacent so that there was no real need to get to grips with the realities of visiting Japan. mapleg just listed some sensible reasons as to why immigration authorities want an address.