This is one of the main religious schisms of GOI, there are a few epic-sized threads about it. Most people agree you need a gunner on really difficult guns like lumberjack, and don't need one on really simple guns like gatling. It all depends on ship and gun loadout.
Even to that you'll find people who argue that one single ammo type and a buff hammer is all you need for virtually every single gun in the game.
My opinion is, if it's not competitive you are free to do however you like. Cooperation is important though and the captain/pilot is the leader. Following orders is important. You could always go to a different ship if you do not wish to do as your captain tells you. Me personally, I will tell them my opinion and what I would do if I feel that what they're telling me to do is bad but I will follow orders nevertheless. The captain is the one who makes the final decisions on the ship.