I had the same question and researched it.
After reading about it, I decided that it is better to leave the car all wheel drive. I think it had something to do with lubrication not properly working when the rear axles are shut down. I think the consensus also was that it really didn't save any fuel.
You may want to double check that.
I think, also, in the manual, it says to not leave the fuse in longer than necessary (in, out) - 2WD no longer than absolutely needed.
but also the fuse holder is empty when in AWD and you insert a fuse for 2WD. they designed this so you could use it for diagnostic purposes. eliminate part of the drive train to help isolate problems. and it is pretty good at that.