In addition to the above, the value used is entirely dependant on what your device is. You want the solution frequency to be sufficiently high, such that the mesh is accurate for higher frequencies, but you also want it close enough to your desired frequency range such that you aren't converging on an undesired mode.
For structures which support not many other modes, a high frequency is generally better, but if you're expecting many modes in your frequency range, a solution frequency close to your sweep range will probably be more accurate.