The instability of oceans jets such as the Gulf Stream and the Kuroshio appears to be dependent upon the distribution of potential vorticity. The formal argument favoring this state of affairs is generally based on linear quasi-geostrophic dynamics from which it can be shown that the necessary condition for instability is that the gradient of the potential vorticity changes sign. In the present investigation the consequences of this result are further investigated by extending the validity of the analysis to the regime of finite but small Rossby numbers. Hereby, the interpretation of the criterion can be generalized to also take into account effects due to a steeply sloping interface, and the relative vorticity of the geostrophically balanced basic state. In this case the symmetric jet is found to have an asymmetric distribution of potential vorticity that exerts a profound influence on the stability. Depending upon the circumstances the ageostrophic terms are found to either stabilize or destabilize the basic flow. As an example of possible applications it is demonstrated that even a vortex sheet becomes less unstable when the interface slope increases.