The effect of the variable surface geometry of a cone on the linear stability of a supersonic boundary layer flowing over it is investigated subject to different quasiparallel flow approximations. It is shown that, if a suitable set of disturbance state variables is chosen for the normal mode analysis, these effects can accurately be accounted for. In fact, a planar coordinate system can be used for the stability analysis of the cone boundary-layer profiles and a simple "correction" can subsequently be applied to obtain an accurate approximation to the spatial growth rates.