The Pro version of the Android iBeacon Library comes with an automatic battery saver that slows down scans when the app is in the background to save battery. The open source Android iBeacon Library also allows configuration of a foreground and background bluetooth scan rate, and provides methods to call to tell the library when the app is in the foreground and the background.
On iOS the battery saving strategy is similar. When no app is ranging for iBeacons in the foreground, bluetooth scans are not performed constantly. The exact frequency of background scans is not published, but I have measured it happening once every 15 minutes on a iPhone 4S with iOS 7.1.
Scanning for specific UUIDs has no effect on battery life. At the OS-level, a Bluetooth LE scan looks for any Bluetooth LE device that is advertising regardless of whether it is an iBeacon or whether it has a specific UUID. The filter for specific UUIDs is handled in software at a higher level.