I don't ever turn it off. The battery lasts anywhere between one and two weeks--this includes intermittent WiFi usage. My device hasn't presented any off-deprivation symptoms so far (like the ones you get when you don't reboot a windows computer after a while).
The reason I don't turn it off is because it takes a few seconds to turn it back on and I'm the kind of person that thinks that someone should invent something faster than a microwave!
I guess the bottom line is that it is a personal choice. No harm either way.