I saw this method when Elizabeth Hartman did her tutorial. I feel like the whole internet went nuts about it. But I’ve never tried it because it seemed like a completely unnecessary step, that would only take longer to put together, and result in bulkier seams and a stiffer quilt. Even lightweight interfacing adds a certain stiffness, that seems unappealing for a cuddly quilt. And spending the time to lay out a million squares perfectly straight, then iron them without nudging any out of place, or needing to slide the whole thing over on your ironing board to get the whole width, seemed like an exercise in frustration. I always felt like as long as I cut my small squares accurately (which is easy to do with a rotary cutter and mat), and used a consistent seam allowance, then pressing your rows, you’ll get perfect seams and corners.
I’m totally not trying to rain on anyones parade, and if other folks have success with it, that’s awesome! Just explaining why I never bothered trying. Especially on something as easy as squares.