I normally wash a dish if I'm already washing our own. However, that's with single guests/families, not multiples. If this was a family get together, I imagine that would be a ton of dishes to wash in addition to their own. If I had a messy dish to take home from something like this, I think I'd rinse the bits off myself quickly (if the sink was needed), or wash it entirely. It's nice to clean up other people's dishes, but I don't think it's an error not to.