I do them all, every time, even if I'm in the same a/c for the third time that day. Things fail, rubbish could get in pitot on landing, FOD could blow into engine intakes, split pins on light a/c can work their way loose. I'm not taking a risk that I don't have to.
And for flying at night, I won't do it if I don't know the area, so I know what should be under me approximately.
[This message has been edited by Lucifer (edited 23 November 2000).]