The Mezzanine

I tip in the US and anywhere else where the guidebook (possibly written by former waiters) says to tip. At home, I round it up if the service is above average and if I'm paying cash, which almost nobody does anymore.

I'm wondering who decided that waiters, of all service categories, need to live on tips. And I also wonder how... say, a train conductor or a librarian would react if I tried to tip them. There might be awkwardness.