Later in the career I grab an engineering officer (propeller symbol) and put him in the damage control leader spot just to have someone with high leadership and mechanical skills but leave the rest empty until I get serious damage. I hacked files to make the observation scope and one tube in each torpedo room indestructible because I got tired of constantly having to head back to base with no repairable scope or tube. Periscopes in particular get destroyed with no hope of repair all too often, and it's always both of them. Guns I don't worry about, they rarely get destroyed, just damaged.
Deck Watch and guns positions are usually occupied, SOP is to use the gunners (Chief Gunner's Mates usually) in the torpedo rooms, but a torpedoman is usually better.
Get a lot of renown and you get a lot of medals, most of them "wounded medal"s, so after the second or third patrol I hand them out to the gunners and deck watch since even if they haven't been wounded yet they eventually will be.
Filling all the slots is an option, matter of personal taste, how much renown you have, and how many 100+ Mechanical guys are available. Beginning the career without any damage control crew I usually grab torpedomen and the two CGMs to fill up the slots when I have heavy damage, but to the best of my knowledge the crew in their own compartments contribute to the repair work. I don't know that for sure but I remember reading it somewhere.