I guess it depends on how you play. I don't use auto-promote, and do all of my upgrading and army formatting in preparation for Barbarossa.. so starting skill level is more important. I agree with the old guard purge though, in my case to get rid of the clutter.
As a rule I don;t use auto promore either because often times it promotes one of your best potentially high skill general before you have a chance to skill him up.
If you train your best Generals to their max skill whule they are a Major General it goes real quick. Unfortanely if you auto promote him to a high level General odds are he will never get near his maximum skill.
The problem I have with auto promote is that every once in awhile I load up as my major puppet to make some adjustments to their builds and research. Then I save and reload as my country. Unfortunately that puts the auto promote on no matter if you want it, or not.
The main trick I have learned is finding the best Major Generals with the highest possible skill potential. (The ones with the ideal ranks of Field Marshall and General 0 and 1). When you train them in combat as a Major General they get experience lightning fast.
The problem with some Generals who might have the highest skill potential of 9 often times their ideal rank is just a LT General and once a a general reaches its ideal rank it becomes like an OLD GUARD in terms of how slow you get points.
I regret not compiling the ideal ranks when I started this project.
I had to go back and do a quickie.
At the bottom of the first post I made a list of most of the ideally ranked Field Marshalls and Generals.