(Orginal post from the 3D Printing Experience blog)
Well, this applies basically to the Prusa i3 and clone style printer more than others as they have the Z and Y axis separate. But many people have had issues with their bed seeming tilted after leveling it to the nozzle. The most likely the reason is the X/Z carriages and rods aren't level or square to the Y rods.
|Notice when the X and Y aren't level (square) it makes the head lean|
By using this Simple tool designed by mangtronix from Thingiverse you can adjust your X-Y to be level and square then level your printers bed to get a more even 1st layer and improve the print quality.
link - Tool to level X-axis of Prusa i3
I also redesigned it just a tad, mostly just made it taller as with my printer his wasn't tall enough to reach once my printer had hit it's end stops.
Link - X to Y Axis alignment/leveling tool
MAKE SURE YOU POWER DOWN THE PRINTER BEFORE USING THIS!!
You can easily bend the rods if you use the motors to mover the carriage and have the tool in place!! It's pretty easy to do.. Disconnect at least one of the motors (they can often produce enough energy to motor the other side with it depending on your mother board) slip the tool onto one of the Y rods, lower the Z/X carriages down to just touching the ledge on the tool. move to other side and repeat. check and recheck each side till they are even. Make sure the resistance when sliding the tool back and forth is equal on both sides. when done. Plug in motor and power up printer. Now level your bed like you normally would. Here's a video on how I do it.
Leveling or Squaring the axises to each other can stop many printer issues like banding in the Z axis on the print (the nuts are binding on the threaded rods because they are twisted) as well as leaning models do to, well the whole printer leaning in general.