Our welcome dinner is immediately after the rehearsal. We have invited all of our guests to the welcome dinner, but only the bridal party is at the rehearsal.
Rehearsal confirmations don't occur until 30 days before the event, so we made ressies for our welcome dinner far in advance (right at the 180-day mark). If you decide to change venues, you can always cancel the current ressie, but unless you're doing a private party, the closer to the prospective date, the fewer options available to book for large parties.
I suggest you book something now so you have something. Then if you need to fix it later, you can always do so.