When did you do your seating chart?

FMIL wants to decide seating arrangements for her family and friends, which is fine, but I want to know who's going where soon since I have to write names and table numbers on all our placecard favors before I assemble them (requires glue) and fill them with candy. She's telling me it's futile to do the seating arrangements now because when she did their bar/bat mitzvahs there were several last minute cancellations and an addition. um, there aren't going to be any "additions" who get seats and food. I've already given the caterer numbers. if necessary I can just take away a placecard for no shows, but I need to have a seating chart for the caterer (who wants a breakdown of who is eating what by table) and I need to get working on the placecards.