We checked in at customs in Ely on our way back yesterday.  Showed our passports and THEN had to fill out a pieces of paper with name, address, etc., etc.  which we thought was redundant paperwork since the attendant also scanned the passports.  The officer took a cursory look in our car...didn't see much, it was so stuffed there was only minimal room for us three passengers to squeeze in.  ;D
I often have not checked in on past trips but have never heard from them that they missed me.  Once I checked in and they were not there, so I stuck my Canada permit copy to the door and left a note "I was here".
Once my dad did a Quetico "walk" and after he got back they called to see if he ever showed back up at home.  Again, no repercussions  :-?