But there's no harm in giving the new one if you can do the maths quickly.
If I'm going to need you on a heading for a prolonged period I will ask you to clarify what the new one is, so any further adjustments can be given as "turn left / right heading ... degrees" instead of "turn left another 5 / 10 degrees"
If, for another example, I need you on a parallel heading against another aircraft, I need to know what one of you is on to match it with the other!
I'm not expecting you to be aware of the traffic situation but giving your new heading covers both bases - I can note it down on my strip (or electronically) if I need to, or ignore it if I don't!