Not that exacting, it is nice to have all the grounds connected for a little noise reduction, but missing a standoff or two isn't the end of the world.
A lot of designs relied solely on the I/O shield for ground and used plastic standoffs. Addtionally, expansion cards will also ground the motherboard to chassis.
Basically as long as the ground plane in the motherboard have a path to ground, everything will be fine. It is mostly for safety anyway, the entire thing could float and still operate just fine.