You max Q depending on who is the enemy mid. Max Q only when you want to harass constantly and the enemy cant completely even the trade like fighting against pantheon or talon or max it if you know that you cant out trade the enemy like fizz.
Max W if you know you can Q->E->W then run away as fast as you can from the enemy like when Ahri misses her charm or champions that relies on a disable/skillshot to trade properly.
Often I max Q first because Kat gets bullied all the time anyway. W is really short ranged and if the enemy is decent then you'll be punished almost every time you try and last hit with it.