Well, the only dinner parties I've ever been to have been hosted by family, so you help with the clean up whether you're leaving after or not. I've had friends over, and they do offer to help clean up, granted my house has a dishwasher so it's not like everyone is washing the dishes. I usually brush off offers for help mostly because I'm tired and want people to leave. Which my friends don't take offense to, they know that even though I love them all I have my limits. Also I become a very difficult conversation partner at that point, because for whatever reason when I get really mentally exhausted I start to speak of myself in the third person, using my parents nickname for me, Nikki.
This is another one of those things where I'm like "Why the heck doesn't anyone just SAY something!?" I mean, how do NTs learn this stuff without someone actually telling them!? Whenever I have problems with things like that, I usually ask my mom and she helps explain it to me, even the very obvious (to NTs) stuff.