Well if you blitz then high taxes would be the best option, no question.

But when I want to play a slower-paced game, I always focus on settlement growth. By the time I can built Merchants Wharfs etc, trade is usually my highest source of income.

Low taxes also give high chivalry generals, which in turn gives a huge boost to growth, especially in castles, and so thats a major factor for the low taxes argument.

However, by the time my settlements reach Minor City status, I usually raise the taxes, as by that time trade and the various buildings you get mean that you can have very high taxes and still see good population growth.