I've never tried it and I've never been to an event where they checked the RSVP. I've been denied entry many times because the venue was at capacity or it was only open to badge holders, but never because I didn't RSVP. I still RSVP at the Filter/Fader events just in case.
I hope I can make it up for the Icelandic Airwaves showcase. That would be a good morning. The Ásgeir Trausti stuff is so chill. Hopefully, it's an electric set.