I have always left the main house battery switched on in most cases. I would turn it off for working on wiring
or devices downstream. My bilge pumps and some alarms are wired before the switch although I can turn those off manually if I wanted for some reason, e.g to work on them.
I always turn off all unneeded circuit breakers when leaving the boat. I leave nav lights on (especially anchor
light) when I will be anchored out at night. I leave the fridge breaker on if I am using it. I rarely turn off the main DC breaker unless working on the panel.
I fuse or have a circuit breaker (both with the proper rating) for every
wire coming off of the battery. I also make sure that every wire is at spec or over spec'd for the most amperage it will need to carry.
I do know some turn off the main DC switch when leaving and that is totally OK is they don't need any circuits on. I would never leave the batteries paralleled when leaving a boat unless it was an emergency
need to charge the batteries together. I don't recommend that unless all the batteries are the same type, etc.
If you don't have adequate fusing and/or circuit breakers or wires in poor condition or undersized then you should certainly turn everything off and plan to improve it all to spec so you can. The way I look at it, if it is too unsafe to leave on at the dock
, then it is too unsafe to leave on when you are on it. The issue then is whether you might leave on a circuit (lights, fridge, propane
, etc.) and then drain the battery when you don't need to, then that is the main reason some will turn off the main switch when leaving. I have done it on occasion but not as a rule