Read the DRV8825 datasheet, detailed description section. 71% is 71% of the current rating of the motor.
BTW you know with a unipolar motor you have to derate currents 70% for fullstep compared to wave mode?
I already wrote, that I know this table. But that's the behaviour of the DRV8825, measured in %. It has nothing to do with the related values of a specific stepper. The stepper datasheet knows nothing of a DRV8825, and also other driver datasheets are different.
The related values for a motor can only be taken from the motor datasheet - not from a drivers datasheet. And the motor datasheets usually only talk about full step. There is no derating. Rated current in the motors datasheet is for full step. Nothing else is written there.
Well try the motor with your current and see how hot it gets and compare with the motor datasheet value I suppose - I'd have thought stepper driver chip makers understand conventions relating to stepper motors.
Of course steppers get hot when the are operated with the rated current. StepperOnline usually talks about 80°C max with both coils engaged with the rated current.
And of course, they will get less hot with your approach. But you will not get the rated torque of the datasheet.
Edit: and obviously it's always a good idea to stay below the rated current if you don't need the full torque 
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Of course they do. But the values in the DRV8825 datasheet are only relative values compared to the max in wave mode. These are not absolute values and they are not relative to a specific rated current of a motor.
If you want to get the full torque with microstepping, the max current in one coil must be greater then the rated current if the other coil is switched off completely.
The DRV8825 controls the current in the coils in such a way, that the sum of the power loss in both coils remains always the same.
You may need to check the issue of wiring, there may be a poor contact.
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