Disconnect battery when parked?

To the people giving me **** I hope you carry a fire extinguisher because I do.
I do too, even in my daily drivers.
Just this Sunday, my cousin came down to look at a big block D-150 I had, he said he wanted it.
I said I just put this fire extinguisher in it this morning, I am keeping it.
He said ok, he just wanted it for the engine and trans, even though I just drove it a few hours earlier.