I always mention to my students that some techniques are a personal choice. There isn’t the right or wrong way. Just the way you feel the most confident or comfortable.
Some people don’t pin at all (I’m getting to that stage, I’m afraid) but I learned to pin horizontally on the seam allowance as guide. If you pin horizontally, always pin towards you so that the pins will be on the right side for sewing.
Pinning vertically is perfect to anchor fabric in place, so they don’t move. I use to match all my seams, notched etc. So my pining is a mix of horizontal, vertical and big gaps without pins.
How about you? What is your preferred method? What technique did you learned first?