There was a time when 10% was acceptable, then it wasn't and 15% was the new acceptable. Then 18% was acceptable. Now it's 20%. Tomorrow it will be 22%.In my opinion, those suggestions basically serve two purposes. The first one is to make the math easier for everyone, because, you know, math is hard. The second is to try to guilt people into thinking 18-20% isn't acceptable anymore, when it is.
It already costs a fortune to eat out. It wasn't long ago when I could take the family out for $50. I got divorced 4 years ago and 4 years ago I paid $70 or so for 4 of us. Now I take the 2 kids and my oldest's boyfriend and it's over $100, plus the tip.