I think the Wifi powersaving mechanisms of the N8x0 are very efficient. No need to turn Wifi off in order to extend the battery life.

I'm mostly using Wifi at home. So I in the first months after I got my N800 I used to put the tablet into offline mode while not being at home.

I stopped doing this already quite a while a ago and honestly I didn't see any difference in battery life.

(Though I should mention, that I have the Wifi Autoscan / Autoconnect features of my N800 disabled. I guess if you enable these additonal features this might increase power consumption.)