the screws should be "firm" and if u try to rock it 360 it shouldnt have any movement.
also remember to do it diagonal at a time (so top left then bottom right then other two) and dont tighten them allway 1 at a time. alternate so you have equal pressure across entire part.
oh..and you CAN overtighten one and it isnt so much "breaking" your MB as damaging the pathways (as they are pretty close to the surface so a nut digging into it from being too tight can cause serious damage)