I think, that if you put the curtain rail only a small distance under the top of the window, it would look odd - it would look as if you had only done it to make your existing curtains fit, which is exactly what you're doing! Is there any way you can let the hems on your curtains down to fill the gap? Or why not find some contrasting fabric which matches something else in your room, and add this contrasting band on the bottom of your curtains, to fill the gap?
Much better, I think, to try to make your curtains fit your window rather than dropping the rail.
