Which ever suit your needs best and the noise level can be tolerated.
I have all mine on medium and it keeps things nice and cool. Just make sure to have good cable management so the airflow is not getting blocked at all and is allowed to flow cleanly through the case.
Where have you put your hard drives? one of the things I noticed that made a huge difference to the temps was to keep them in the first bay at the bottom. This allows for the middle bay to push clean cool air straight into the case over the GPU,NB & CPU!