Although it is listed in the nec to attach lighting fixtures to the grid, I have been told by more than one fire dept not to. If my fixtures are independantly supported and the fire dept needs to get to an above ceiling fire. usually with a hook or back of an ax. they don't want lights and loose wires falling on their heads. 95 % of the work i do is commercial, and always support mc by buidling structure i.e. purlin, beams etc. until it reaches its target. for 2 reasons it looks neater, and it will pose no hazard to falling through the ceiling.
