I am not certain but I believe the tilt adjustments the firmware makes is to the tower tilts NOT THE BED DC will have to confirm this mind.
FYI: From the calibration page:
Bed should be at right angles to the towers in both X and Y directions
Actually, this isn't essential, because RepRapFirmware can compensate for bed tilt. Unfortunately, auto calibration can't easily distinguish between bed tilt and differences in tower separation. One approach to getting the bed level is to mount it on 3 equal-length spacers attached to the lower horizontal frame. If you are using metal corners, the horizontal frame should be at right angles to the towers. If you do need to make any adjustment, you can stack washers or shims on top of one or two of the spacers.