Many people like to start their first city in Evony with a 50% tax rate. This allows them to have a decent population while producing some gold to help them fund their early research levels (and hire heroes.)
When you have a 50% tax rate you’ll run a 50% loyalty which means you can have 50% of your total population. This means you won’t have a lot of extra population. You’ll have to keep your cottages upgraded so you have some idle population when you build new structures. However it will provide a decent amount of income from taxation to hire heros and perform research in your Academy. You also won’t have to worry about your people revolting like you might if you try to run a 100% tax rate. Also since you aren’t building a big army at first you won’t need as much idle population as you might if you were later in the game.
This is why 50% tax rate is considered the best tax rate for starting in Evony.