For something big I would chase up RSVPs. For something informal I was having at home I'd just assume they weren't coming. If they still turned up I would make a comment about not expecting them!!
For our wedding I had to chase up quite a few RSVPs, it's so annoying. How are you supposed to assume certain people are coming? Why would you bother sending invites if this was the case? My DHs aunt didn't RSVP & even when my MIL called her she still wouldn't answer yes or no. Her reason - she didn't know what the weather would be like on the day!! WTF has that got to do with anything?