I've read that tipping usually is a no no. What are the exceptions?

I understand no tipping in restaurants. At hotels it was suggested that you could give a small gift to the concierge at a hotel. Far as I know other employees including bus persons don't get tipped. Someone suggested that rounding up was okay in taxis. I'm guessing that means to the next 100 yen.

But what about tour directors on organized tours and private tour guides? Is it proper to tip porters at airport, train stations and elsewhere? Who else?

All the tipping info was scattered. Maybe a tipping sticky would be a good idea.