I do not use auto positioning loads because like others have said, it only frustrates me and causes issues. For me it's not just lighting fixtures, but video walls too. I can be editing something and moving stuff around and not even notice something moved, and then all of the sudden trims have changed and deck fixtures are attached to nearby truss. Great concept, poor execution. I turn off "Auto-position loads when created" AND "Auto-position loads when moved" and also set the pick radius to 0" for good measure. As mentioned in a previous post, that is found under spotlight preferences. That keeps things automatically snapping to trusses.
Then I use the "Manage Loads" on the rigging tool set to add or remove. I don't know how to do several fixtures with one click. This method you have to click each fixture. Just get it on the truss and then use this tool to add or remove. Note, that I always make my trusses hanging positions before I do this. I don't know if that has anything to do with why that wouldn't be working for you.
Anyways, this new workflow is helping me, and keeps me from pulling my hair out and cursing at my screen. Hope that helps.