Pretty sure NFC is going to be the next fad that doesn't take off. I was with someone who used it on their phone, it took her longer to get her phone out, find and open the app and pay than it did for me to pay with plastic. And it's just one more place to store my credit card info where it could potentially be stolen. I have zero desire to use it.
The right thing to do is have the app open while you're in line and ready to go, not open it up when the cashier tells you your total.