If you finish off a normal tile you should always get an extra item, this is not true for invader/ghost lairs.  Also the higher the level the tile the better the extra item can be.  One of the best times to find low rss tiles is right after an event as people do small farming to avoid tile hits.  Also some people leave 30 K on lv 6 tiles because they forget that a full lv 6 tile is 1,036,800 and not the 1 m that is listed.  By cleaning up these tiles not only do you give yourself a better chance of getting a good bonus item you are doing a service to your kingdom because new lv 6 tile won't regenerate into you kingdom until the old ones are completely farmed and disappear from the map.