I've actually done this after a month after owning my PSM. It works and is very noticable. Make sure you check which board you have before making changes. Copied from: http://help.printrbot.com/Guide/Adjust+Motor+Current+(Printrboard+Rev+F)/153
Below is a copy for an easier read, but its for the Rev F version of the printrboard.
Stepper motor current is adjusted by sending commands to the printer.
Note: Commands are case sensitive.
M907: Sets motor current as a percentage of the full-scale adjustment (ex: M907 X30 Y30 Z30 E30 sets all motor current levels to 30% of the full-scale reference value).
M909: Reads the present motor current setpoints (similar to M501).
M910: Saves motor current setpoints (similar to M500).
2 Step 3
Use the M907 command to set a mid-level value (50 is a good starting point). For example: M907 X50 Y50 Z50 E50.
Use manual movement commands (larger values work best) to move the motors.
Incrementally adjust individual axis down 10 points at a time until they stop moving. Example: M907 X40, and then M907 X30.
When the motor stops moving reliably (skipping steps), move it back up 10 points, and move to the next axis.
Run M910 to save the new values.
Run M909 to verify that the values were saved.