Using Motor Steps Instead Of MM For Z Axis

For step-based layers, you don't need to change how slic3r works, you just need to set your layer height to a value so that the math works out the way you want.  The stepper motor has 200 full steps per rotation, with each full step subdivided into 16 microsteps, yielding 3200 microsteps per rotation.  For the default 18tpi z-rod, the firmware is set to 2268 steps per mm, so if you use a layer height of .299824, you should have 680 steps/layer no roundoff at the microstep level, or with a height of .303351 (688 steps/layer), the stepper motor will always fall on a full step (or at least the same microstep) for even more consistent results.

That being said, I don't think the moire effect you are seeing is likely caused by the z-axis movement, as any effect would be orders of magnitude too small and would only in one dimension. Roundoff or microstepping is often blamed (mistakenly IMHO) as a cause for banding effects, but moire effects are commonly associated with the granularity in the movement of the extruder motor.  I'd try adjusting the output voltage on your controller board on the line going to the extruder motor, tweaking it while sending extrude or retract commands until the motor runs smoothly.  I did this on mine and that removed the moire effects I was seeing.