If you ever get a chance peruse the "Uber" message boards and see what the drivers say about tipping. They are absolutely ruthless with downgrading folks who don't tip. Mine has also gone down after experiencing a dirty/filthy car or the ride was less than five or ten minutes.
It actually soured my approach to tipping. I know not all of them are like this, but the vocal few was enough for me. I still use them, but I only tip if the ride was above and beyond. If my rating goes down, it goes down.
Saw that in the forums as well... Am I the only one offended by that? Seems incredibly offensive to me to downgrade someone just because they don't tip. I'm already against tipping but this really pushes me over the edge. (I tip but feel very obligated to do so).