I've got a quick question about the grids in viewfinders with respect to the rule of thirds. I'm specifically looking at the Nikon grid on a D300 but I guess it is common across all Nikons (I have a D40 at the moment which doesn't have a grid at all). What I see is the viewfinder is divided into quarters both horizontally and vertically - is this in any way related/helpful to the rule of thirds? I was wondering that maybe it was quarters because it might become thirds once the image is later cropped to a 5:4 format but that's probably wrong.
Would you use this grid other than aligning the picture? Does it have a use for applying thirds or is it more of a distraction. Seems odd that this rule is generally accepted and the manufacturers put a grid on the screen that doesn't meet the rule.