As with most defences it depends a lot on the terrain and whom I'm defending against.
Since shelter bunkers are cheap and more realistic in any numbers than armed bunkers I buy 1 or two per trench line. I tend to put Hq units under this kind of cover Zugs and Kompanies.
I also find armour fare better dug into trenches, however if you move them DON'T try driving them back in as they will be disabled more times than not.
Generally speaking these kinds defences are more effective against Russian Assaults. Also don't forget to move the "sheltered" troops into the trenches once the enemy is within rifle range.
There used to be a bug years back that allowed you between battles in a campaign to buy bunkers as part of the core force and in one campaign I had enough shelter bunkers too put all Hq Gruppe under cover in a defence. Came in handy from 43 to 45 on the Ostfront.
