Disconnecting the battery is never a good idea. The biggest reason to get used to not disconnecting the battery doesn't apply to Toyota products, ... Yet. It can cause a real lot of misery and expense on Volkswagen, BMW, and General Motors vehicles. They have a lot of tricks designed in to cost unsuspecting owners money after the sale. A lot of Volkswagens end up being skidded in park onto a flat bed truck for a trip to the dealer to have numerous computers unlocked.
For your vehicle it's mainly fuel trim data that will get erased. Those "look-up tables" will start to be rebuilt as soon as you drive the car, and you won't notice anything happening. If your radio needs a security code to be entered, you must know it beforehand.
You can find "memory saver" devices that plug into the cigarette lighter or power outlet, but those only work if those outlets remain live with the ignition switch off. If that switch has to be on, there's no way that little 9 volt transistor battery can run all the computers and other stuff. Also, once you disconnect the car's battery, don't open the doors or the interior lights will turn on. Disconnect an under-hood light too.
I use a small battery charger to maintain computer memories but you have to be careful that the clamps don't fall off, and especially that the positive battery cable doesn't touch anything metal on the car.
December, 11, 2012 AT 2:57 AM
Forgot to mention too that many cars today have computers that need up to 20 minutes to go to "sleep mode", and during that time they can draw over three amps. The batteries in memory savers can't supply that much current so you have to wait for those computers to time out.