Battery (really solar panel) Disconnect Question

Had a C40 controller destroyed when one of my 14 yr old batteries went south, so yes it is advisable to disconnect the panels fist, thats the way I was tought when I worked in the solar for 3 yrs. in Arizona and was working for 1 of the best @ Imperial Dam. and all I had was a 80 watt panel charging while the rig was in storage at the time. Just my thought's.