I've never felt awkward hitting the no tip button. I didn't even realize that people felt it was an obligation for the longest time. I would just hit the "no thanks" button and think nothing about it. I also never got the impression that the person working the register thought any less of me.
I tip delivery drivers and waiters. That's about it.
I will only give the server a cash tip if its been great sevice, and if the food has been amazing a separate cash tip for the kitchen. Im not giving you a tip for just doing your job. Tips should be for being good at your job, not just doing your job. Thats what your employer is for.