personally i just max out the POs and officers straight away, i like to have one officer of each skill and add the others later on(or at the end of my first patrol with commander). so 1 WO,torpedo expert, machinist, repair man and a medic. helmsman is a pretty useless skill but its nice to have along with radioman and medic for completeness. my PO layout is usually close to 10 torpedo men, 2 machinists,2 repair men and 2 radio men(Plus whatever you want to fill the last guys out with). you dont really need a flak gunner, deck gunner or medic PO as higher officers fill those bars very well.
i do find that commander is a cheat sometimes because i can train the best crew in the entire german navy in just one patrol although crew transfers do help drop your crew to a normal level. also i hate fatigue so i turn it off but if you play normal then i would advise maxing out all your men. i tried having just enough men to fill quarters so they would always be rested(excluding maintenance crew like watchmen,engines etc ) however it can leave you short at times when you take damage with an inexperienced crew or wish to reload aft and fore of tubes
hope this helps you out
edit: oh yea and save the renown for a nice IXB when it comes out or a VIIC