I've done it both ways on several jets. Pros and Cons for both ways. Just some of my observations below:

Right behind the turbine is hotter and gets better smoke most of the time especially at or near idle power. It is possible to torch the fluid if especially if you are spooling up the turbine when turning on the smoke. Fluid is more likely to spill, drip, or leak inside the fuse if you have a accident or malfunction.

Out the back of the tail pipe gives good smoke as long is it is hot enough to vaporize the fluid. However, several times I've seen smoke density drop off as thrust is reduced toward idle and almost no smoke at idle even though the fluid is spraying out into the exhaust stream. Running the smoke tube between the walls of the pipe is a great way to preheat the fluid. Having the smoke nozzle at the rear eliminates the possible mess inside since all the fluid is sprayed behind the fuse unless there is an internal malfunction.

I always drill weep holes usually before and aft of each former where any fluid that may spill inside can drain out and not puddle. Seen a couple of bad fires after some puddled fuel got lit after a bad start. Just my two cents worth.
Gary