One answer I recently heard on this from an SMT assembly machine
supplier was that two local fiducials can actually make theta
location of the device worse. The reasoning was that the
distance between the two local fiducials is very small. The
error in theta location due to the camera resolution was higher
than the error in theta you get going off the widely spaced 









         global fiducials. Their take was that you use the board global 
         fiducials for theta location of fine pitch devices and a single 
         local fiducial for x,y correction of the device. Localised x,y 
         drift of the board location was much more likely than localised 
         theta skew. In addition the extra time needed to locate a second 
         local fiducial would make a serious impact on high speed placement 
         throughputs. 
         
              I have no direct experience of this myself and I have not 
         done the calculations on theta error, but it makes some sense and 
         might be worth considering.
         
               Jeremy Drake
         
         Design to Distribution. Stoke on Trent. England