Test the thickest point
Do not choose from the flat denim edge if the needle must cross a folded hem, pocket or patch overlap. Fold the project and test the place that offers the most resistance.
The needle should enter with controlled pressure and come out without bending. If you need to grip it with excessive force, reduce the layers or choose a more suitable size.
Keep the eye kind to the thread
A heavy denim needle with a rough or overly tight eye can cut sashiko thread during repeated passes. Match the eye to the thread and inspect it before starting the full repair.
Thread a short sample and stitch several rows. This reveals compatibility sooner than waiting for a break after the garment has already been marked.
Plan around seams
Seams can deflect a needle and create an uneven stitch path. Slow down at those crossings and consider a shorter needle when there is not enough room for a long rocking motion.
A well-chosen needle helps, but no tool replaces safe handling. Keep the point directed away from your supporting hand and store it securely between sessions.