I have the same issue with my case, only came included with the 9 mounts. Same one is missing, the board has the hole, but nothing on the case. I installed my gfx cards and there doesn't appear to be any stress or bending on my board at all, even without that particular mount.
Put it in the case, and see if there is any weight issues for that section, but you should be fine.
Sounds good. I figured it wouldn't really be an issue, but was just getting a bit OCD having ten holes in the case, ten holes on the board but nine screws. You spend over $100 bucks on a board, you get a little agitated to see them skimp on a screw that costs pennies. I even went down to the local PC shop, but unfortunately this one is a bit longer than the regular two short and long standoff sizes they had.