"Don't round off even if the question tells you to" is the advice I give my learners.
In fact, they can round off but ONLY when they've written down every single digit on their calculator first.
This way they're guaranteed to be awarded the marks. If they incorrectly round on the answer line without the calculator display written down first getting that mark is much more of a gamble.
Here's an example showing how it works:
This question clearly asks candidates to give their answer to 2dp:
But if we look at the mark scheme the A1 is for an answer in the range 13.85 to 13.86, not actually for an answer of 13.85 or 13.86. It's a subtle but important difference :
If the candidate is a proficient rounder then there is no problem, but how often do they make a mistake, especially when under pressure?
A simple error such as rounding to 1dp (13.9) will take them out of the answer range and therefore they will not be awarded A1.
Writing down the whole calculator display first stops this being a problem as they will be awarded the A1 for 13.85640646 and a subsequent incorrect attempt at rounding will be ignored.
Other tips to stop your students from losing marks can be found here.