Pololu drivers. That current limiting pot they have.
Mine have a small pot on them, I had a look at the data sheet and it discusses setting current limiting using that pot to change a Vref.
The A4988 datasheet that is, I can't find anything about it's use on the Pololu website.
So.. my question is.. have you had to twiddle that pot at all ? or is it not critical in this application ?
this is about the best you can find: http://reprap.org/wiki/Pololu_stepper_driver_board#Tuning_motor_current
in my experience with the HPC3020 laser: just put them somewhere in the middle and it is about fine... steppers and mechanics are small and light so no problem.
just one thing to keep in mind: when you have no cooling element on them: make sure they do not get too warm, even after using the laser for half an hour or so, mines can still easily be touched with my fingers...