My friend, the awesome Geeky Hostess has a new column on manners for geeks that she is publishing every Monday. You really should visit her every Monday for easy to apply advice, but today she hit on a topic that is particularly relevant to parents - the RSVP.
For things like gymnastics centers, indoor play areas, bounce centers, etc. parties are many times charged a flat rate. For example, a party may cost $200 for up to 10 kids. The parent planning the party is going to get charged $200 even if only five children attend. In this case it's imperative to RSVP if you can't make it because the space that was intended for you can be offered to someone else.
We frequently do birthday dinners at family friendly restaurants. If I make a reservation for 20 people are 10 show up, the restaurant has planned to staff for 20 they're going to be annoyed. If 30 people show up and they have to smush into a space for 20 everyone is going to be annoyed.
As far as the etiquette of the manner of RSVP'ing, I don't really think it matters how you do it as long as you do it. Recently, due to vacation and craziness at our house we neglected to RSVP for a wedding This is bad*. I know how annoyed I get planning an five year old's birthday party, I can't imagine how much stress a bride goes through when she has RSVPs floating out there with no reply (sorry Anne!). So as soon as I recognized what we had forgotten - only a few days had passed. I texted the groom and let him know. He was cool. And now everyone is happy. Just RSVP.
*I would like to point out that this isn't even the first time something like this happened. I actually sent my RSVP to another wedding years ago, and the couple never received it. Again, all was remedied with a phone call. Just communicate!