My sims tend to wash dishes in the bathrooms because, as justproud2b says, I make the bathrooms as easy-access as possible. (Three sim pregnancies in rapid succession taught me that bathrooms CANNOT be too close or too numerous.)
I also find that often one of the kids is eating in the bedroom talking to the other two while they work on their skills or do homework, so when one of the sims goes around collecting dirty dishes to clean up, especially if they start downstairs, they collect ALL the dishes and usually end up upstairs in the kids' room. So I think for me this is an indirect result of multitasking.