This contribution shows that some useful properties of LQG-balanced realizations of continuous-time systems are preserved by applying bilinear transformations. This fact suggests that LQG-balancing of discrete systems and its approximation can be performed by using the typical schemes successfully adopted for continuous-time systems. In the paper the approximation of discrete linear time-invariant systems by using the singular perturbation approximation approach for LQG-balanced realizations is analyzed, showing its usefulness in approximating discrete models, including unstable ones, for closed-loop aims.