There's still no obligation to tip whatsoever. After your ride, when you next open the app (which could be days later) you are presented with a screen asking you to rate your driver (this already occurs). I think if you give a 5 star, then the option to give a "compliment" is given, which is similar to a badge or a Facebook like.
As well, now, a tip button is also presented. You don't have to click on it if you don't want to.
I will never tip with zero guilt.