When you start up your system or do a new job, surely the first thing you do is home the machine (at shut down you home the operator ) The initial gantry alignment error will either not change if the steppers lock in the same direction or each or one of the steppers can be up to 1.8 degrees out. At worst this will give 3.6 degrees or 1/100 of a revolution. Divide the screw pitch by 100 and you get the gantry skew. For a 10mm pitch this is 0.1mm . Now you decide can you live with the chance of this until you Home the machine and correct the skew.