They're purely targets of opportunity for me, though I love popping tincans. It depends on the tactical situation. If there's more than one, and if weather is calm, then I tend to lay off the DD and go for merchies or capital ships. In rough weather the bad guys will have a much harder time spotting you on sonar, so take that into account. Torpedo tracks are also much more visible in calm, daylight conditions. Thus, easier for them to avoid. They also provide a "HERE I AM" notice.
When stalking a pair or more of DD's, I favor shooting the last ship in the row, as it takes a minute or so for the fish to hit the target, I get a little extra time to make good my escape as it takes a few minutes for the other DD to initiate a search.
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If it's a fair fight, then you didn't plan it properly.
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