GoingForGold:
I just read that you should choose the voltage 10 to 20 times the voltage rating of the motor. So 30-60V, do you agree? Will the specifications change if I dont operate it at the voltage specified in the sheet?
Only if using a high performance bipolar motor for extreme speed (a CNC rig for instance). You'll then need the suitable constant-current buck-converting drivers. The high supply voltage enables the driver chip to overcome large back EMFs when the motor moves fast.
If you aren't looking for much speed its much simpler (cheaper) to use unipolar motors driven by ULN2803 or similar - then the PSU voltage should be a volt or two more than the winding's rating (6V supply for 5V windings for instance - for 12V the difference doesn't matter).