tedster - 4:47 am on Jun 23, 2012 (gmt 0)
I really don't think that advice applies any more. It used to be that you could see ranking changes pretty quickly, and by making one change at a time you learned about what change had what effect. But Google today is a lot more complex - with ranking changes bubbling up and down over time, even though the actual cause may have occurred at one specific time.
I know that I personally dropped that one change at a time approach a while ago. If you're making basic, on-page changes, then there still might be something to say in favor of one at a time. But most of the sites I work with are making all kinds of changes all the time anyway - there's just no time built into that process to wait around!
That said, if I need to change the URL structure, the entire main menu, or something really major like that, then I would definitely carve out a period of say 2 weeks or so to launch the change and watch at least the first layer of effects.