Are these going to be inspected by a building inspector? Because if they are, i would suggest yes they need to be fitted. I'd hate to be picked up on that after an install.
It depends on the installation itself IMO, how much insulation is in the ceiling (if any) ? Its down to judgement, although i consider it good practice to fit them and i suggest you should no matter what the installation compromises of - its better to be safe than sorry.
I don't have a copy of the building regs to hand, surely it must mention it in there somewhere?
Im sure i once read somewhere that it's the ability of the floor to remain intact for a certain period of time (in the event of a fire) - which is the ultimate deciding factor as to whether to fittings need to be fire-rated or not. DO NOT quote me on that though.