You can configure Netflix for Android to be able to stream or watch instantly over the 3G network and WiFi or only over WiFi.
If you do not have an unlimited data plan, or a big enough one, you may want to configure Netflix to work only over WiFi Networks. One movie can be more than 200 MBytes.
To configure Netflix on Android to only stream over WiFi. Enter into the application, and scroll over your recommendations, down to the end. You will see, at the bottom a button to enter into your account information. And there you can configure it to only work over WiFi networks.