When do B List Invitations go out?

For those who have 'A' and 'B' lists for wedding guests, when do you send out the 'A' list and when do you send out 'B' lists? Do you need separate RSVP cards?

For ex: Say you have a min of 120 and you invite 150 guests. Say by RSVP date you have 145 responses and only 100 say 'Yes'. Do you go to the B list and send out those invites or just call people to ask them to come even though they didn't get a formal invitation in the mail?

Once we get to a certain designated # of no's we will be sending out b-list invites a couple at a time.

Our minimum is 100 and say we send out 125 invites. If we get 30 no's then we will send out 5 invites etc.

lol... i better start thinking about that too
my invites go out this fall