Plus, the actual strain of the boot process may actually completely offset the very, very meager gains a shutdown might have netted you, especially if you essentially use your device every day.
That, and the boot time is just plain annoying, no matter how fast it is (try cold booting a Kindle, it's, err, quite the test of patience ;D).
Definitely on the "never ever shut it down" camp

. It'll shut down on its own if it needs to (i.e.,: when it runs out of juice ;p).