Most residential roof trusses are engineered to hold 10 PSF Dead load on the bottom cord. A load that does not move. Drywall, insulin, ductwork, electrical, lights, etc. Most residential roof trusses are engineered to hold 0 PSF live load on the bottom cord. Stuff that moves. People, furniture, storage, etc. Some times you can find the ratings somewhere on the truss. It should be 4 numbers like 30-7-0-10. Top live, top dead, bottom live, bottom dead.
Do people store stuff on top of the bottom cord, yes. Should they, probably not. If you stay under 10 PSF, you are probably fine. But drywall and insulation weight more than you think.