I leave my Oliver
plugged in all the time and never worry about it. As long as you have a smart charger built in, or a quality battery tender, you're good to go.
A fully charged battery is quite freeze resistant, but I can't remember down to what temp they are OK.
Batteries will live longer if they are maintained at a float charge and not just left disconnected and then charged occasionally. Plus you have to pull it out and re-install if you bring it in.