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.
Typically the taxi driver will round up the fare to next one dollar.
So no tipping is required except for good service.
Only for 5-star hotels with toilet attendant that hand you clean towel and
turn on the tap for you; otherwise, there is no need to tip the attendant.