Trade buildings get ridiculously powerful at level six if you have plenty of provinces in which to build them and reap the +1% trade efficiency bonus. I tend to build production building early on because production and trade efficiency are low and taxes make up most of your income, so workshops, constables and treasuries are vital. Later on I concentrate on docks, counting houses and trade buldings with a few provinces dedicated to army level six for buffing up manpower and force limit.