[llamabeast]

Obviously, over an infinitely long game you would do best to tax at 100%.

However, in practice income during the first 30 turns is worth A LOT more than income during turns 31-60 (for example). This means it's hard to give a mathematically meaningful answer as to what the best approach is. Plotting some graphs and/or just playing with different strategies should give you some idea of what strategy you want to take.

Playing as Man you have powerful troops but weak magic. I think it might make a lot of sense to overtax at the beginning and create large armies in order to get a lot of territory early on. Hopefully you'll have enough gems and forts (for research mages) by the midgame that you'll be able to remain strong when magic becomes dominant.