Can I use 7.2v lipoly for 12v stepper motors?
Yes, but at roughly 60% of the motor's rated capacity.

Looks like 6V is another common stepper voltage, so check to see if any will meet your performance needs. For a mobile, battery-powered application you'll want to pay attention to the power and torque densities (and compare to some de-rated 12V options). Since a stepper driver has to include current-controls (preventing the thermal failure mode), you can run a 6V stepper at 7.2V with no problems.

Steppers (also DCs and BLDCs) have a roughly triangular maximum operating region which trades off between speed and torque. Changing the supply voltage shifts the entire line, as shown in the figure. Note that some manufacturers will provide performance charts with rectangular regions - this is their rating for continuous operation, formed by taking the original triangle and chopping off the high current (thermal failure) and high speed (bearing failure) areas.