I use "tipping" in my Hacker News app Hack. Basically users can tip an amount they pick. Would such "no tax on tips" apply to that too?

If it's free for all users, and you don't provide any benefit to those "tipping", it's already an untaxed gift in the US, if no individual gifts more than $19,000, and even then, the gift giver would pay any taxes. Tips require a customer relationship to exist.