Never heard it, and it sounds more like a guide for people who have no actual ability to drive, like the "stop for 3 seconds at a stop sign or it doesn't count" BS. You should already be monitoring your mirrors, so you should already have an idea if someone is over there or not. With that in mind.
Turn signal on as soon as you decide it's time to merge*
Check blind spot mirror.
Check regular mirror.
If both are clear, merge.
If one of them isn't clear, wait for lane to clear, then merge.
*Note: People who turn on their turn signal a split second before merging have no clue how a signal works.
ha! or the person who signals halfway between lanes! i think there just doing it so the road safety program will record that they signaled