In addition to Scott's very correct & comprehensive post:
For exit, show window, etc. boxes in the T-bar ceiling, get the Caddy 512 series bar hangers designed for this purpose, they only require 1 support wire.
For any surface mounted fixtures, track, etc, the Caddy IDS series hangers are the way to go. They have a means for connecting a support wire to the top, and clip right around the grid.
Although it's not required unless you're above a fire rated ceiling, if you take a can of spray paint and spray your bundle of ceiling wires before you put them up it will make the wires you install easy to recognize, and should get a smile from your inspector.
Oh yes, also don't use toggles for support in the hard ceiling. Support any fixtures with screws to the structure.
Have a grand time!
