Jonathan,
I have found tracer dye to be helpfull in checking septic systems but it is only one conponent of the inspection.
On one occassion the dye surfaced in the sump pit. The repair company found the pipe had cracked at the exterior wall of the fountation and was entering the drainage tile. I would not have found this without the dye test.
On more than one occassion the dye surfaced in the yard or front ditch.
Once the system is pumped the dye test is useless until the stank fills back up. It is also useless if the house has been vacant for an extended period of time. I then include a comment that problems with the system may not become apparent until daily use is continued.
I use the liquid but have used both and think its just a personal preferance.

Jim Thompson
HomeFront Inspection Services