one thing you need to remember is it is not the holding torque you need but the torque at a given speed.
In most cases the holding torque is totally irrelevant.
Also if you under rate the current this will only really effect the torque at the lower speed range, and will have little effect at the higher speeds as this will be lower than the torque you would get at lower current.

Dont forget the lower the current means the higher the inductance and that means you need more volts to force the current through the copper. So spending more money for a 3.5A motor will in most cases give you less performance than using a 4.2A motor and running at 3.5A.
Without the volts you are choking the driver and you will not get a good performance.
People always forget about volts, and with a stepper system it is very important.