Quote - I gather that with Poser 6 and up, a figure (or a pice of conforming clothing) no longer needs to have existing INJ channels in order to be able to accept deltas via an inj pose?
Am I understanding this correctly?

If you use PMD based INJ poses, then you're correct that channels can be added on the fly.

Quote - I'm trying to figure out if I make a figure, and want to create a morph add-on for it later, do I need to create the empty channels, or is this P5 and earlier kind of a thing?

As PMD support (i.e. binary morphs, and new keywords) was introduced in P6, then for P5 and before, the channels either have to exist (for updates/upgrades it means a new CR2) or you use Dodger's "wrapper method" (which is similar to DAZ's EXP but without the need for an updater program).