This might be a stupid question, but I have seen several youtube videos describing different techniques - notably whether the guitar should be under tension or not, when using the rocker.
I have been using the rocker on a perfectly flat neck (i.e. notched straight edge, truss rod adjustment) with no tension to find high frets. Is this not the correct way to go about it?
Whatever your technique, if you are successfully finding and correcting high frets, you are by definition using the tool correctly. I just use a 6" machinists scale, and I guess I usually have the guitar at pitch.
I don’t think it is necessary to have the neck under tension when doing this. By definition you are measuring over very short spans of the fretboard - comparing the height of a fret with the two adjacent frets. In such a short interval the presence of a bit of relief or neck bow from string tension would make no significant difference.
It is a different story when you are using a sanding beam to level the entire fingerboard. Then, obviously, the strings are off the neck but it is important to use the truss rod to remove any bow in the neck and level frets on a dead level board ( like you describe in the original post).