Yes, my dentist was sure. He explained that rinsing before you brush loosens food particles and buildup, making brushing even more effective. There's even a market for the stuff, dubbed "prerinse." If you're a mouthwash-aholic like I am, though, it's best to choose a brand (like Crest Pro-Health) with little to no alcohol content since the alcohol can have a drying effect on your mouth.
The only downside of the pre- and post-rinse: I still love that just-rinsed feeling after brushing, so I end up using mouthwash both before and after I brush my teeth — which means going through twice as much mouthwash.