They may take the liberty of rounding up the fare to the nearest dollar figure. Some however, will round the figure downwards and make you pay a few cents less than the actual fare.
You should also remember that there are road and tunnel charges and HK$5 is charges for each piece of luggage put in the trunk.
Although most restaurants usually put on the bill a 10 per cent service charge , this sum does not go to the waiting staff - the restaurant keeps it. Therefore,it is customary to leave some tip of the 10 per cent : even a small sum is appreciated