You realize, of course, that the difference between five and six is one side or the other of Blvd St Michel? My initial comment apply equally to the new area -- they're both great.
As an aside on the use of 'arrondissements':
They're simply municipal adminstrative districts and have little bearing on anything else. Nobody, except for folks who have only been to Paris a couple of times (this includes guidebook writers who, I strongly suspect, have only ....), uses them for anything else. Folks that live there know which one they live in since it's part of their postal code -- but probably couldn't place more than five or six others on a blank map.
Paris is a bunch of villages that have grown together, so people will tell you thay live in Montmartre, Montparnasse, or Montreuil.
Directions are generally given in terms of a metro station; e.g., Rue Vignon, four blocks northeast of Madeleine.
Sometimes directions may also be given in terms of a prominent landmark, e.g., Notre Dame. Confusion can ensue since there's a metro stop named Notre Dame des Champs (they're a mile and a half apart).
Thus endeth today's lesson on the political geography of Paris. :)