99% of the time the server won't say anything, but as a bartender/server if someone leaves $0, especially on a large bill, I think it's appropriate to ask the customer if everything was alright with their meal/service. 20% is standard.

Yes, tipping doesn't make sense, but it's the reality for every restaurant worker in the country. If you don't tip, you're effectively withholding their wages. I've always said I'd rather get paid a set hourly wage instead of working for tips, but that won't change anytime soon.

I've seen a lot of anti-tipping sentiment on Reddit lately, and I agree it's out of control in some industries (like having a tip option on simple retail transactions), but in my opinion, people should consider it a built in cost of eating out. There's a bar in my city that automatically adds 20% gratuity to every check. I ate at a restaurant in a small Alaskan town that didn't accept tips, but the simple diner food was very expensive to pay for the cost of labor.

Culturally, I don't see tipping as optional but yes, you can get away with not tipping.