Couple questions about Wifi

[mustangmelish]

Alright, so I know that Wifi uses less battery than 3G, but if I'm out of Wifi range, should I turn Wifi off? Will it constantly search for a new network if I go out of range (and kill my battery), or will it idle until I manually try to connect to another network/access data? I'm usually in a place where there is Wifi, but I don't know if I should be turning it off when I leave to go somewhere else.