Look it depends upon the flights. On a wide body with a ton of tango fares you'll be able to book seats together at t-24... probably. They won't be preferred nor near the front. Plus side, zone 5.

Aisle and window preferred fill up first. So if 3-3-3/3-4-3 you do get lucky (which I doubt) your preferred seats will be middles.

If you leave it really late you'll get GTE and increase your odds of being IDBed. Even if you get a few dollars it likely won't compensate for a huge potential delay (like say somewhere AC flies to 3 times a week) and dealing with agents that are, at best, limited by AC IT. Being Tango you'll be bottom of the list for finding a flight for.

I understand she's not your wife, but the assuage happy girlfriend happy life can also apply. Get your card out.