It is the custom yes.. I think, if you have worked a job like this (as many of us had growing up) that you will always try to tip when the option is available.. to do otherwise is a big fuck you to the service. If it's not a service though, that is a luxury (like having someone cook and serve for you), then it's not necessary.

Apps may be predatory, but .. that doesn't mean the solution is not to tip. Also, I'm old, maybe the culture is changing and food or drinks just fall from the sky like uber delivery magic.