I usually come to think about my commanders a bit into the game when I create a new army and thinks "hey, I want Rommel here... Where was he? I know I've seen him somewhere". Went to the ledger, found the Division he was assigned to and promoted him. Actually gave me quite a few famous names with high experience because they had been on the front line in Poland, France, and Turkey.
Student got II. Fallschirmjägerkorps and had 4 stars,
Rommel got Heergruppe B with 5 stars.
Model and Guderian also had their fair share of stars, like many others...
And so on...
Unless you bring in your HQ's for frontline duty, assigning some commanders you want to level up to divisions is a good way to get really good commanders.
But personally, I would like the higher commanders to get a small portion of the experience from all units under their command.