Reporting Services provides some nice options to allow user’s to feel like they are interacting with the data, by expanding lower levels of detail when they click on the plus (+) sign in the report’s tablix. You can also set the initial Toggle state of report items as expanded i.e. (-) sign. There is a catch though in that when you Show your initial state as expanded, the items may expand correctly but the toggle sign appears reversed (i.e. + instead of -).
Nice solution to the problem demonstrated Here.