I rarely buy fish, but when I do, I just keep them in my QT system for a month to make sure they eat and don't carry parasites. I don't treat unless I see something.
Corals are another story. I break the corals off the base, then dip them in Flatworm Exit and do a Lugol's dip. Interceptor scares me, so I don't use it as a preventative (never used it as a cure either, but I have it on hand.) I seem to have the "eagle eyes" for red bugs, so I just keep a close eye on the corals in my quarantine tank for about a month to six weeks. If there's any amount of the base left on the coral, I super glue over it to prevent any algae from surviving.