If it is a unit cost (for example) then generally work to the nearest cent (i.e. 2 decimal places). If it is a total cost, then generally work to the nearest dollar.
In Section A it will be clear from the choices of answers how many decimal places are required.
In Section B they are not usually worried about rounding – the marks are mostly for the workings. If you are doing it the right way then you get most of the marks (whatever answer you end up with). For that reason it is vital that you workings in Section B and neat and clear for the marker.