But what about this on the Uber site:
When using uberTAXI (requesting a ride from a cab via the Uber app, available in select cities), drivers will input the metered fare into the Uber driver application. A default 20% of the metered fare will be automatically added and paid to the driver as a gratuity. You can select the percentage amount of the gratuity by signing into your account at uber.com
then clicking the ‘Payment’ link at the top."
Automatically adding 20% seems like plenty of tip. The X and Black fares are more than double the Uber taxi rate, so those rates seems sufficient as well.