The current can be reduced. It depends on the mechanical holding torque how much current is needed for that.
Keep in mind that microstepping is for smooth operation, avoiding resonances, and does not provide accurate steps or constant torque.
The standard practice is to drop the current by a factor of two or so when the motor is stationary,reducing motor power consumption/heating by four or so. The stationary torque is always more thandynamic torque, so this torque reduction is not normally a limiting factor.Completely powering down means the motor loses authority (unless you have a worm drive tohold position), and could move an arbitrary amount.
If you use 3:1 gear with 16 microsteps, can you use a 3*16=48 (or more) ratio gear with full step ?A gear with 3:1 is only little for accurate things. Google returns many results for : stepper gear 100