I don't think that's necessary. I started my updates on the way home from work today. I took the long way home, so it would have plenty of time to finish the software update (since you can't turn the car off until that's complete). By the time I arrived home, the software update was finished and the map update had been running for about an hour & was at 35%. I parked it in the garage & shut it off just like any other day.
Several hours later, I jumped in the car to run a quick errand, and a few seconds after starting it I was prompted for the second map update disc & the status was at 51%. So it appears that it continued to work on the update while the car was shut off. The battery wasn't noticably affected.
When I got back home from my errand the status was at 57%. After another hour of sitting in the garage shut off it's up to 80%, so I expect it'll be done about hour from now.
So, to make a long story short, once the map update gets to 5% you can shut off the car & walk away while the update runs unattended (other than to swap discs after a couple hours).
A few other observations:
The software update took about half an hour (so taking the long way home turned out to be an unnecessary precaution for me). It spent only a few minutes reading the first disc, and all of the remaining time was on the second disc.
The touch screen is not usable during either update, except as needed by the update (such as entering the license key).
I wasn't able to use the radio while the software update was running, but I could use it during the map update. The volume control buttons on the steering wheel worked, but the channel change buttons did not - I had to use the center console buttons to change radio channels. I didn't try any of the other media options, I assumed they would not work without the touch screen.
The environmental controls didn't seem to be affected by either update.