Motherboard Install (Used 6 screws instead of 9)

My Antec 900 came with 6 brass mobo offsets (or whatever they're called), preinstalled. When I put in the mobo, I didn't think to add the three extra offsets. Thus, my mobo, which normally would use 9 screws, is held in with only six. However, those six are under the heaviest items (ie: the graphics card and the CPU cooler). It's only the outer 3 screws that have not been installed. There really isn't anything there for them to support except for the board itself and one stick of ram. I did notice that when I installed that one stick of RAM, the board flexed a little bit, but other than that there isn't any detriment.

I took a look inside the machine, and I would literally have to take the entire thing apart in order to get those last three screws in. I'm wondering if they're absolutely needed.

6 is probably fine and since it's already installed you'll risk more damage by taking every thing apart to install 3 screws then what you will by not having them there. The only issue I could imagine with no having those 3 installed is if you constantly add and remove IDE cables on that end of the board. Without support the board will flex which will stress the board when pushing the IDE cable in. But really how often will you do that?

If your computer starts acting buggy, e.g., turning off and on randomly, this could be the cause because without the correct number of standoffs there can be short