I only return to base properly on two occasions:
1.) Im trying to advance the game clock. I dont want to be in 1940 forever.
2.) I still have a deck gun to use on the single merchants. Cant attack a convoy, but i still got shells to lob over at tommy. Of course this is coming from a captain who sinks everything red. If i track a sound contact and pop the scope and it turns out to be a fishing boat, tug, or trawler - he gets a broadside of AA guns and heavy flak.
Otherwise no, dont have to return to base and its not really worth it.