However, over time, the battery strings, being not identical due to all kinds of reality things, will diverge in their capabilities.  The time-honored way to deal with this is to "equalize" the batteries from time to time, which means to charge up the batteries until they start gassing. Since they are no longer accepting a charge when they are gassing, all battery strings will eventually become much more equal in their state of charge (some will stop accepting charge sooner than others, but eventually, all will).

I recommend to use Hydrocaps (a device that replaces your regular battery caps and will recombine the H2 and O2 gas into water so you don't have to add water so often), to use a battery desulfator (try www.homepower.com for a circuit or google it to purchase one), and to equalize about every 3 months.  Of course, your mileage may vary, and you should equalize and check water often until you get a good feel for your system.

As far as how often a battery (cell) short-circuits, I have never seen it happen, but as has often been said, batteries seldom die, but are often murdered. Um, oh wait, yes, in fact, I have murdered a battery which short-circuited, by letting my truck sit for months at a time, many times, without starting it and recharging the battery (eventually, I bought a solar charger which solved the problem). You can tell that a cell has short-circuited if you charge and condition a battery and the ramp and/or gassing voltage is less then you expect by 2.2 volts (ramp) or 2.5-2.6 V (gassing).

I'd love to give you an URL which tells all about batteries, but I've found few good ones, and I can never seem to find them when I need them.