Fake currency issues greatly diminished once the "most valuable note" became worth not very much. While it is always possible, figure that the most widely available note in circulation is worth less than US$10, so if you get scammed on your change from that, you're out no more than a few dollars. And the odds of that are quite low. After hundreds of taxi rides over the years, I've had one attempted phony taximeter charge (called him out on it and paid him less) and maybe two, max, drivers that wanted me to change $ (possible scam) - and both were many years ago.

I typically use Cabify, and pay cash. If I were to use UBER, unless my cash was running low, I'd also use cash.