It's also true that a battery that has been at rest for a winter unconnected and charged up will be fine. Add a charger during long time storage and you have the possibility of over-charging it or boiling it dry. As a teen I worked in a marina where we would pull and store over 50 batteries and once charged and checked for electrolyte levels, they remained un-connected and untouched from October to April or May.
Back in Post #11 twinboat mentioned that a battery at rest won't be outgassing dangerous levels of hydrogen. If you've been cranking the starter or had it on a fast charger, large amounts of hydrogen gas can build up. It's something to always keep in mind, but it's normally not a dangerous thing to disconnect or hook up a battery. If you are still concerned or afraid, disconnect the other end of the ground cable, connect to the battery, then reconnect the cable and make the circuit complete away from the battery. You could also just pull the large, 80-120 amp master fuse to open the circuit if you are sure the battery isn't connected by secondary wires.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI