I see that Tanchelyn had some good advice,

dragging and whilst keeping the mouse button down, you can hold the TAB key
so you can temporarily move your selection to the exact spot you want it to.

That would also work. 

guides ...

That is a possibility ... however, looking back to the reference picture from the Ansel Adams "Camera and Lens" Black cut-out frame concept ... please keep in mind that I would be trying to find a 'hot spot' (point of interest) in an image ... to maximize its value ... and to minimize inclusion of unwanted elements ... like waving the cut-out frame around before taking the picture. 

The 3-way control: (a) aspect ratio; (b) moving the selection within the image; (c) being able to change the size of the selection ... is what I was looking for, 

Thanks ...

--
Martin