Tips have not been discussed formally at my outfit, but the idea is brought up between crews on a not too infrequent basis.
The consensus is that the vast majority of the crew members would like to tip. However, that same vast majority is under 35 years of age, and many no longer carry minted or printed currency. I too have found that as time goes on, I am using plastic more often and thus have less money for tipping on me.
One item of food for thought would be if the world moves forward with any of the cryptocurrencies; tipping could move back into the fold as one would be able to "bump" the tip over to the driver. Of course, these currencies would have to stabilise first, as I can't imagine anyone would freely give away any part of a Bitcoin right now.