Let me be a bridezilla for a minute. Not even a bridezilla, because I've thrown parties for people recently, and I've had parties thrown for me. Maybe more "Ms. Manners" than bridezilla. But people, listen:
If you are invited to something that requires an RSVP, you should RSVP. No one writes that to look fancy. No. They're doing it because they don't want to spend money on your meal and gifts if you're not coming. Also, when you decline, it's nice to say thanks for the invite. Someone made a decision to invite you. You didn't need to get that invite.
For our wedding, for example, there is a stack of invites that haven't been given out because we don't have room, yet only one person from my extended family has RSVPd. I know they know whether they're coming. But they don't care to tell me. Ugh. And for our shower tomorrow only one person RSVPd without being prompted. Ugh. I feel like such a chump because the person throwing the party just spent 2x the money & effort that was necessary.
So yeah. RSVP.