Unchecked cameras are aligned, but their coordinates don't participate in equations, so they are not disturbing the bundle adjustment.
if you have cameras with high accuracy and others with low, you can set specific values for each: One by one by doble click in accuracy cell and then enter value, or in groups by multiple selection, then right click and modify... accuracy.
if accuracy is similar in XYZ then enter a single value i.e. 0.005
if horizontal accuracy is higher than vertial, you can enter the two values separated by "/", i.e. 0.005/0.015
if you had independent x,y,z, accuracy estimations, you can enter the three values in the same way. i.e. 0.02/0.03/0.05
Regards
GEOBIT