Potentially still an issue if drivers can see if you tip or not (with some services, such as Postmates, Amazon Prime Now, they can) and give riders who don't tip 1 star or something.
Or even if it happens after rating, they might stereotype certain parts of town due to how people there tip on average and then underserve those places. The great thing when I first used Uber was how in some parts of town it was impossible to get a taxi but a Uber would always come.