from

http://www.cs.uiowa.edu/~jones/step/

which is the first result if you search in google for "stepper motor"

fourth paragraph in introduction:

If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.

I am not saying this will occur, but if it does, it will acumulate over time and make your measurements worthless, unless you have a way of "resetting" the stepper..

I am also not saying that you will be able to triangulate if you take into account the possible slip of your stepper, I only say that your measurement of the angle between two targets will be more accurate if you calibrate (reset to a known position) before starting your measurement.

regards
Patrick