A properly engineered and constructed truss should not sag I would think. But they are very prone to going down in windy conditions if not sheathed. I alwaqys breathed a little easier when getting the sheathing down for that reason.
Sag? No. Break, fall down, etc? Sometimes if they are not braced properly. But I suppose if they were up, unprotected in the weather for a long time the cycling of rain (moisture) and drying might cause warpage.
I know that - I wanted to know if you were talking about the sheathing for the roof (topping the trusses) or if you wanted to know if the bottom chord would sag without being sheathed with something.