I usually start in a sugar boat, and if I survive long enough I take the new command. After 4 or so patrols in an S-class I can appreciate the new Gatos and Balaos.
If I were to start off in a fleet boat from Pearl, though, I would probably stick with it, unless I was in a Porpoise or something and was offered something really nice.
I usually did the same thing in SHIII, starting off in a Type II and moving up as soon as I could.