A poor bit of advice is 'you should always be doing something on a nav'. Sorry, but that's bull****.
The point is that many people find that if they
think have nothing to do, they have probably forgotten something. So maybe the advice should be that if you think there is nothing to do, have a rethink and make sure everything that needs to be done really has been. If it's all been done, then sure, relax for a bit and save your brain for when you need it, but make sure you know when you next need to be doing something.
There is also a danger in trying to be too far ahead of the aeroplane. During my initial sim training on a turboprop, my buddy and I were so intent on being ahead of the aircraft that we were dicking around getting an ATIS for the destination when we should've been doing climbing transition checks
.