Place drywall joint at sagging truss center or overlap?
Your trusses are defective and done very wrong and would only be used atop columns, supporting beam or bearing wall. Call the truss manufacturer for replacements that are built right, solid and seamless sticks on the bottom.

Sagging before even installing the drywall's load is a disaster ready to happen!

After that correction, then you can put your drywall joint on the truss for very wide butt-joint treatment. Or even better is, to put your joint between trusses and very shallowly "V" back-blocking framing for a tapered treatment.