It really depends on your class/build/skills and gearing level.
For most of my builds I like to lower them as far as I can without dying a lot. This is because setting them really high will cause your bot to go into avoidance mode more often. Avoidance mode takes priority over most aspects of combat. If you set them all to max, you will see what I mean.
Therefore having them higher than needed means more time running around avoiding and less time fighting. Often times, avoiding too much can lead to more deaths since you're not casting needed spells and elites get more affixes down. And it obviously can cause bad clear times.
My suggestion is to set the things that kill you a lot high, then lower them every so often until you find an optimal setting. If EVERYTHING is killing you a lot, then you probably have some gearing issues or are out-GR-ing your gear/skills/build.
And OFC there are some things you always want on high like Molten Core which will 1-shot most non-ZDPS builds at high GRs.