That's a good question nutty! I've always just tested my tank when the urge struck me, but I could be persued to change my testing habits with a good explanation of when and why to test at particular times.
Usually ph will be lowest right before the lights come on and highest before they turn off.
I've never heard of the others making a difference.
Could be though.
I would say test in the morning and test at night to see what the difference is. If the goal is stability, as long as you test at the same time everyday you should be fine.
