In a restaurant
If your bill is delivered by a waiter, it's very likely that
10% service charge is included in your bill, so tipping from HK$2 to round up
to next $10 may
be expected (depends on the service). More for exceptional service or
good restaurants and expensive western cuisine restaurants.
Taxi
Typically the taxi driver will round up the fare to next one dollar.
So no tipping is required except for good service.