The rule I've always gone by (wrt MC etc) is we don't attach our stuff to any thing that we didn't install to attach our stuff to; or to the the equipment it terminates into. Simple. PITA, but simple.
For LV I'll bend the rules and attach to pretty much anything available to attach to that **isn't** being used to support *another* trades work excepting the ceiling support wires themselves (and excepting job specs that *specifically* say no to that).
Clear as mud?
(the Hilti bang sticks work best)