Same here, don't see the idea about wasting torpedoes to sink DDs when you can use them on a large tanker or freighter
As for warships, I would go only for light cruisers and above. Unfortunately when I meet them it's usually in bad weather, or they are too fast and off my course to catch them...
I normally don't sink them while they are in harbours are it's like shooting fish in a barrel, not much sport (except if it's a target of a specific mission order), and because historically U boats were not meant to raid harbours.