The most effective tax rate is 0 or 100.

0% is to be used most of the time, as it means that the attacked needs to spend a couple extra attacks to cap your city, giving you more time to come online or for your alliance to reinforce.

If you are, for whatever reason, low on gold, u can TEMPORARILY change your tax rate to 100% to survive the emergency. Only do this if you absolutely need the gold to sell in market/avoid losing heroes/pray.

Your gold, if needed, comes from npcs, you should be perfectly capable of farming lvl3-4 (possibly 5) npcs by the time you're out of beginner's protection.

Having 50% (and similar) tax rates while offlline is an excellent way to wake up with one city less one day.