This happened to me a few punts ago. Of course I washed her tits - they must have had lots of mouths slobbering over them and I wanted them to be squeaky clean.
She laughed and helped me wash my hands, and since we were over-the-top play acting, she made a point of inspecting my nails.
So yes, I think she's intending you to wash your hands, but you have a blank cheque - soap in hand and no instructions, so wash her tits, back, armpits - it's your buck, have fun.
I'm sure you are not the first guy with a handful of soap wondering what to do. She's probably running a poll with her roomate as to pooner reactions.