I know that it is good for him to do this. He also has been asking for some specific toys. He already has daily chores that he does but we have added a few optional money chores for him to do so that he can earn money to buy the things he wants.
Unloading the dishwasher is a money chore at this point but he is usually willing to do it. In fact he has never refused. I haven't told him it is optional.
I have him unload everything and for the bigger bowls and such that is up higher he just puts them on the counter for me to put away.
What do you do to make it easier for your kids to help?
For more ideas see:
Making my Life a Little Easier @ Passionate Homemaking
Creating a House with Built In Character @ Home and Harmony