Once I handed a $20 bill to a cab driver and told him to "keep the change" and he falsified a receipt.

Thanks, but I'm going to tip through the normal tipping mechanism with my normal payment method to ensure that everyone stays honest.

And that probably includes the honest GrubHub drivers who invariably ditch my drink if they decide I haven't tipped high enough in the app.

cash is the normal tipping mechanism, universally accepted all around the world.

I do it as often as I can, which is sadly not often because I rarely carry cash. We do, for a variety of reasons, tend to frequent places that are ethically run though, and often know everyone involved fairly well. But, yes, as long as we have tipping as a significant basis for restaurant workers' income, cash tips.