I think it's a balancing act, and it's a pretty appropriate one. Of the leaders you listed, Patton was never more than an Army commander in the field (technically, he commanded First US Army Group before the invasion of Normany, but that was a dummy formation that was a giant deception), Montgomery did his best work as an Army commander (8th in North Africa), and Zhukov was shuttled between Fronts to shore up critical defensive situations or to lead key attacks. To add another, Rommel did his best work at lower command levels; by the time he was an Army commander, his highly personal leadership style was impairing overall effectiveness. If you keep your key commanders at low level positions (Corps/Army), they can have a very significant effect on that unit, but everyone else has weaker leadership. If you promote them up, they can provide a weaker impact over a broader area. I personally look a lot at admin ratings. High admin ratings tend to equal higher position, since the kinds of things that the higher units do (especially transferring armies between fronts) really need those admin rolls to succeed. I see people use Zhukov or Guderian as high commanders and I cringe. Those positions are just too far away for those leaders to have major effects. Frankly, some of the issue with "Major General Nobody" comes down to how histories are written; very few historians research figures beyond the flamboyant 'names', so they wouldn't know a Manton Eddy from a John Lucas from a Lawton Collins (or a Mainstein from a Guderian from a Hoth, to bring it back to WiTE land).
To sum up the 'who goes where' question - high political and administrative skills get someone promoted up; lack of those will keep them down, regardless of other skills. Combat skills are everything at corps level, and initiative is a great thing with your mechanized formations; do not forget that (and that's one of the big areas the Germans get an advantage in, IMO).