Since Selena is a reiner too I like to practice doing a lot of softening, but I don't slam her face into the ground. And when we are working on barrels, unless she is trying to nose out and take my reins away (Dominant little pony she is) I will leave it completely be and let her do it herself. If she is being particularly hot or explosive I will make her round up while we work because it makes her think.
I do believe in doing it when warming up, but not just headset, a lot of long, low, round, softening work. Bumping up into the bridle, getting them on the vertical and engaging their hind end and rounding their back, as well as counter arc circles and leg yields in a soft frame.
This is especially important with Selena since she does have a rather dangerous history, I use that to get her mind working a little before I give her her head and let her choose her own frame on the pattern.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.