Inclusion of planar layered media Green's Functions (GFs) is a major issue in the computational efficiency of full-wave models derived from integral formulations. These GFs may be decomposed into quasi-dynamic and dynamic terms. In a wide range of practical applications, the quasi-dynamic terms may be given in closed form. This paper proposes two criteria to establish when the complete GFs may be approximated by the quasi-dynamic terms. These criteria are based on simple relations between frequency, line length, dielectric thickness and permittivity. If these criteria are satisfied, the inclusion of the GFs into full-wave integral models is straightforward and the overall computational cost is strongly reduced. The proposed criteria are verified through a benchmark test case. The model is then used to perform a full-wave analysis of the power lost in a microstrip, as a consequence of the excitation of parasitic modes, such as surface and leaky waves.