If you have completed braking prior to corner entry/turn-in, it's not trail braking.
It's the MSF Slow-Look-Press-Roll cornering process, whereby slowing is done before you turn-in (and using both brakes, as de facto good habit). In practice, it's a seamless process as you execute this process but slowing and turn-in are discrete "steps" and separate from one another.
This is considered by most safety experts as the preferred way of speed adjustment prior to putting the motorcycle in a cornering attitude.
This said, I believe being able to use your brakes without exceeding traction when you are at lean, in a corner... is a useful tool in the bag. Whether you call it trail braking or something else is going to be immaterial when that surprise downhill decreasing radius turn sneaks up on you and you have to shed more speed than you anticipated but no longer have the time and space to keep braking and turning separate.
As for front brake vs. rear brake... either way, care must be used so one doesn't exceed the delicate balance of traction as much of it may already be consumed by cornering. Consider though, which brake offers more potency and the implications of weight transfer during deceleration/braking and that may help sway you as to which one to favor more.
