Intra-aneurysmal hemodynamics such as wall shear stress and complex flow structures have been implicated as one of the important factors on the growth and risk of rupture of an aneurysm. In this study, the sensitivity of intra-aneurysmal blood flow dynamics to the shear-thinning rheological model is investigated by using the idealized geometries of a basilar tip aneurysm with two representative anterior-posterior (AP) tilting angles ($2^{circ}$ and $30^{circ}$). By choice of different rheological models, time-averaged hemodynamic factors such as wall shear stress, oscillatory shear index and relative residence time exhibited only minor effects. However, highly unstable flow present in idealized aneurysm model with $2^{circ}$ AP tilting angle facilitated an evident change in the instantaneous local flow dynamics with a considerable increase in effective viscosity. Nevertheless, the distinct hemodynamic phenotype, which characterizes the gross intra-aneurysmal flow pattern, was independent of the choice of rheological model. This result suggests that the shear thinning viscous effect is of secondary importance in the gross hemodynamics in a basilar tip aneurysm but is appreciably enhanced on the instantaneous hemodynamics with unstable complex flow structures.