I'm interested to see the replies to your question, as I am trying to determine the exact same issue.
My first impression was to get the controls as close as possible to the operator, while he was in a driving position. But after thinking about that for awhile, I thought that it might be better to have them out of the way - after all, what kind of implement controls will you have back there which are so time-critical in adjustment that you want to have control in hand while moving and while also 'fine tuning' the position of the implement.
So anyway, that's where I stand today. Get them out of the way and maybe even behind you. When you need to activate the controls, you stop the tractor, reach back, make an adjustment, slip back into driving position and check to see if you have to do that over again. Depending on where you are, you might be able to just slow down but keep moving while you reach back.
I constantly have my hand on the loader control when I am doing that work. But my rear implements are kind of set in place and stay there.