Winterization is a bad word in Texas these days...
I had always tried to keep my deep cycle batteries above 80% at all times, deep discharge for prolonged periods can apparently impact longevity/# of charging cycles. I didn't have any rule of thumb other than to charge them back up every month or two.
Keep in mind this was not for performance reasons - I'm sure for starting or running accessories even an 80% charged battery probably can get the job done. I did this really to preserve the life of the battery as long as possible so I wasn't buying new every 3 years.