Well, I calibrated both the extruder and the Z axis, and it didnt get rid of the ripples - it simply centered them (so that the back is no longer smooth, and the ripples are the same depth on all four sides). I'm beginning to suspect uneven heating.

All 20mm cubes are, well, 20mm tall - heated or not heated bed.

I know PID has something to do with this and the fact that, despite having more than enough power supply, my bed won't heat above 80c, but again - no simplified "For Dummies" guide to what means what when adjusting PID.