Our expense policy accepts a paper taxi receipt. If I'm in a part of the world where cabs are commonly paid for with cash, I'd have no problem submitting a receipt including the tip amount.

But I don't think I'd submit a line item called "Uber receipts". That looks weird. There is no paper receipt. There's the standard Uber receipt that everyone is aware of by now. (I submit a screenshot of that.) I'd end up just not expensing the tips. It would drive me away from Uber on business trips, perhaps to Lyft or some other service.